typescript constructor assignment

Background Reading: Classes (MDN)

TypeScript offers full support for the class keyword introduced in ES2015.

As with other JavaScript language features, TypeScript adds type annotations and other syntax to allow you to express relationships between classes and other types.

Class Members

Here’s the most basic class - an empty one:

This class isn’t very useful yet, so let’s start adding some members.

A field declaration creates a public writeable property on a class:

As with other locations, the type annotation is optional, but will be an implicit any if not specified.

Fields can also have initializers ; these will run automatically when the class is instantiated:

Just like with const , let , and var , the initializer of a class property will be used to infer its type:

--strictPropertyInitialization

The strictPropertyInitialization setting controls whether class fields need to be initialized in the constructor.

Note that the field needs to be initialized in the constructor itself . TypeScript does not analyze methods you invoke from the constructor to detect initializations, because a derived class might override those methods and fail to initialize the members.

If you intend to definitely initialize a field through means other than the constructor (for example, maybe an external library is filling in part of your class for you), you can use the definite assignment assertion operator , ! :

Fields may be prefixed with the readonly modifier. This prevents assignments to the field outside of the constructor.

Constructors

Background Reading: Constructor (MDN)

Class constructors are very similar to functions. You can add parameters with type annotations, default values, and overloads:

There are just a few differences between class constructor signatures and function signatures:

• Constructors can’t have type parameters - these belong on the outer class declaration, which we’ll learn about later
• Constructors can’t have return type annotations - the class instance type is always what’s returned

Super Calls

Just as in JavaScript, if you have a base class, you’ll need to call super(); in your constructor body before using any this. members:

Forgetting to call super is an easy mistake to make in JavaScript, but TypeScript will tell you when it’s necessary.

Background Reading: Method definitions

A function property on a class is called a method . Methods can use all the same type annotations as functions and constructors:

Other than the standard type annotations, TypeScript doesn’t add anything else new to methods.

Note that inside a method body, it is still mandatory to access fields and other methods via this. . An unqualified name in a method body will always refer to something in the enclosing scope:

Getters / Setters

Classes can also have accessors :

Note that a field-backed get/set pair with no extra logic is very rarely useful in JavaScript. It’s fine to expose public fields if you don’t need to add additional logic during the get/set operations.

TypeScript has some special inference rules for accessors:

• If get exists but no set , the property is automatically readonly
• If the type of the setter parameter is not specified, it is inferred from the return type of the getter
• Getters and setters must have the same Member Visibility

Since TypeScript 4.3 , it is possible to have accessors with different types for getting and setting.

Index Signatures

Classes can declare index signatures; these work the same as Index Signatures for other object types :

Because the index signature type needs to also capture the types of methods, it’s not easy to usefully use these types. Generally it’s better to store indexed data in another place instead of on the class instance itself.

Class Heritage

Like other languages with object-oriented features, classes in JavaScript can inherit from base classes.

implements Clauses

You can use an implements clause to check that a class satisfies a particular interface . An error will be issued if a class fails to correctly implement it:

Classes may also implement multiple interfaces, e.g. class C implements A, B { .

It’s important to understand that an implements clause is only a check that the class can be treated as the interface type. It doesn’t change the type of the class or its methods at all . A common source of error is to assume that an implements clause will change the class type - it doesn’t!

In this example, we perhaps expected that s ’s type would be influenced by the name: string parameter of check . It is not - implements clauses don’t change how the class body is checked or its type inferred.

Similarly, implementing an interface with an optional property doesn’t create that property:

extends Clauses

Background Reading: extends keyword (MDN)

Classes may extend from a base class. A derived class has all the properties and methods of its base class, and can also define additional members.

Overriding Methods

Background Reading: super keyword (MDN)

A derived class can also override a base class field or property. You can use the super. syntax to access base class methods. Note that because JavaScript classes are a simple lookup object, there is no notion of a “super field”.

TypeScript enforces that a derived class is always a subtype of its base class.

For example, here’s a legal way to override a method:

It’s important that a derived class follow its base class contract. Remember that it’s very common (and always legal!) to refer to a derived class instance through a base class reference:

What if Derived didn’t follow Base ’s contract?

If we compiled this code despite the error, this sample would then crash:

Type-only Field Declarations

When target >= ES2022 or useDefineForClassFields is true , class fields are initialized after the parent class constructor completes, overwriting any value set by the parent class. This can be a problem when you only want to re-declare a more accurate type for an inherited field. To handle these cases, you can write declare to indicate to TypeScript that there should be no runtime effect for this field declaration.

Initialization Order

The order that JavaScript classes initialize can be surprising in some cases. Let’s consider this code:

What happened here?

The order of class initialization, as defined by JavaScript, is:

• The base class fields are initialized
• The base class constructor runs
• The derived class fields are initialized
• The derived class constructor runs

This means that the base class constructor saw its own value for name during its own constructor, because the derived class field initializations hadn’t run yet.

Inheriting Built-in Types

Note: If you don’t plan to inherit from built-in types like Array , Error , Map , etc. or your compilation target is explicitly set to ES6 / ES2015 or above, you may skip this section

In ES2015, constructors which return an object implicitly substitute the value of this for any callers of super(...) . It is necessary for generated constructor code to capture any potential return value of super(...) and replace it with this .

As a result, subclassing Error , Array , and others may no longer work as expected. This is due to the fact that constructor functions for Error , Array , and the like use ECMAScript 6’s new.target to adjust the prototype chain; however, there is no way to ensure a value for new.target when invoking a constructor in ECMAScript 5. Other downlevel compilers generally have the same limitation by default.

For a subclass like the following:

you may find that:

• methods may be undefined on objects returned by constructing these subclasses, so calling sayHello will result in an error.
• instanceof will be broken between instances of the subclass and their instances, so (new MsgError()) instanceof MsgError will return false .

As a recommendation, you can manually adjust the prototype immediately after any super(...) calls.

However, any subclass of MsgError will have to manually set the prototype as well. For runtimes that don’t support Object.setPrototypeOf , you may instead be able to use __proto__ .

Unfortunately, these workarounds will not work on Internet Explorer 10 and prior . One can manually copy methods from the prototype onto the instance itself (i.e. MsgError.prototype onto this ), but the prototype chain itself cannot be fixed.

Member Visibility

You can use TypeScript to control whether certain methods or properties are visible to code outside the class.

The default visibility of class members is public . A public member can be accessed anywhere:

Because public is already the default visibility modifier, you don’t ever need to write it on a class member, but might choose to do so for style/readability reasons.

protected members are only visible to subclasses of the class they’re declared in.

Exposure of protected members

Derived classes need to follow their base class contracts, but may choose to expose a subtype of base class with more capabilities. This includes making protected members public :

Note that Derived was already able to freely read and write m , so this doesn’t meaningfully alter the “security” of this situation. The main thing to note here is that in the derived class, we need to be careful to repeat the protected modifier if this exposure isn’t intentional.

Cross-hierarchy protected access

Different OOP languages disagree about whether it’s legal to access a protected member through a base class reference:

Java, for example, considers this to be legal. On the other hand, C# and C++ chose that this code should be illegal.

TypeScript sides with C# and C++ here, because accessing x in Derived2 should only be legal from Derived2 ’s subclasses, and Derived1 isn’t one of them. Moreover, if accessing x through a Derived1 reference is illegal (which it certainly should be!), then accessing it through a base class reference should never improve the situation.

See also Why Can’t I Access A Protected Member From A Derived Class? which explains more of C#‘s reasoning.

private is like protected , but doesn’t allow access to the member even from subclasses:

Because private members aren’t visible to derived classes, a derived class can’t increase their visibility:

Cross-instance private access

Different OOP languages disagree about whether different instances of the same class may access each others’ private members. While languages like Java, C#, C++, Swift, and PHP allow this, Ruby does not.

TypeScript does allow cross-instance private access:

Like other aspects of TypeScript’s type system, private and protected are only enforced during type checking .

This means that JavaScript runtime constructs like in or simple property lookup can still access a private or protected member:

private also allows access using bracket notation during type checking. This makes private -declared fields potentially easier to access for things like unit tests, with the drawback that these fields are soft private and don’t strictly enforce privacy.

Unlike TypeScripts’s private , JavaScript’s private fields ( # ) remain private after compilation and do not provide the previously mentioned escape hatches like bracket notation access, making them hard private .

When compiling to ES2021 or less, TypeScript will use WeakMaps in place of # .

If you need to protect values in your class from malicious actors, you should use mechanisms that offer hard runtime privacy, such as closures, WeakMaps, or private fields. Note that these added privacy checks during runtime could affect performance.

Static Members

Background Reading: Static Members (MDN)

Classes may have static members. These members aren’t associated with a particular instance of the class. They can be accessed through the class constructor object itself:

Static members can also use the same public , protected , and private visibility modifiers:

Static members are also inherited:

Special Static Names

It’s generally not safe/possible to overwrite properties from the Function prototype. Because classes are themselves functions that can be invoked with new , certain static names can’t be used. Function properties like name , length , and call aren’t valid to define as static members:

Why No Static Classes?

TypeScript (and JavaScript) don’t have a construct called static class the same way as, for example, C# does.

Those constructs only exist because those languages force all data and functions to be inside a class; because that restriction doesn’t exist in TypeScript, there’s no need for them. A class with only a single instance is typically just represented as a normal object in JavaScript/TypeScript.

For example, we don’t need a “static class” syntax in TypeScript because a regular object (or even top-level function) will do the job just as well:

static Blocks in Classes

Static blocks allow you to write a sequence of statements with their own scope that can access private fields within the containing class. This means that we can write initialization code with all the capabilities of writing statements, no leakage of variables, and full access to our class’s internals.

Generic Classes

Classes, much like interfaces, can be generic. When a generic class is instantiated with new , its type parameters are inferred the same way as in a function call:

Classes can use generic constraints and defaults the same way as interfaces.

Type Parameters in Static Members

This code isn’t legal, and it may not be obvious why:

Remember that types are always fully erased! At runtime, there’s only one Box.defaultValue property slot. This means that setting Box<string>.defaultValue (if that were possible) would also change Box<number>.defaultValue - not good. The static members of a generic class can never refer to the class’s type parameters.

this at Runtime in Classes

Background Reading: this keyword (MDN)

It’s important to remember that TypeScript doesn’t change the runtime behavior of JavaScript, and that JavaScript is somewhat famous for having some peculiar runtime behaviors.

JavaScript’s handling of this is indeed unusual:

Long story short, by default, the value of this inside a function depends on how the function was called . In this example, because the function was called through the obj reference, its value of this was obj rather than the class instance.

This is rarely what you want to happen! TypeScript provides some ways to mitigate or prevent this kind of error.

Arrow Functions

Background Reading: Arrow functions (MDN)

If you have a function that will often be called in a way that loses its this context, it can make sense to use an arrow function property instead of a method definition:

This has some trade-offs:

• The this value is guaranteed to be correct at runtime, even for code not checked with TypeScript
• This will use more memory, because each class instance will have its own copy of each function defined this way
• You can’t use super.getName in a derived class, because there’s no entry in the prototype chain to fetch the base class method from

this parameters

In a method or function definition, an initial parameter named this has special meaning in TypeScript. These parameters are erased during compilation:

TypeScript checks that calling a function with a this parameter is done so with a correct context. Instead of using an arrow function, we can add a this parameter to method definitions to statically enforce that the method is called correctly:

This method makes the opposite trade-offs of the arrow function approach:

• JavaScript callers might still use the class method incorrectly without realizing it
• Only one function per class definition gets allocated, rather than one per class instance
• Base method definitions can still be called via super .

In classes, a special type called this refers dynamically to the type of the current class. Let’s see how this is useful:

Here, TypeScript inferred the return type of set to be this , rather than Box . Now let’s make a subclass of Box :

You can also use this in a parameter type annotation:

This is different from writing other: Box — if you have a derived class, its sameAs method will now only accept other instances of that same derived class:

this -based type guards

You can use this is Type in the return position for methods in classes and interfaces. When mixed with a type narrowing (e.g. if statements) the type of the target object would be narrowed to the specified Type .

A common use-case for a this-based type guard is to allow for lazy validation of a particular field. For example, this case removes an undefined from the value held inside box when hasValue has been verified to be true:

Parameter Properties

TypeScript offers special syntax for turning a constructor parameter into a class property with the same name and value. These are called parameter properties and are created by prefixing a constructor argument with one of the visibility modifiers public , private , protected , or readonly . The resulting field gets those modifier(s):

Class Expressions

Background Reading: Class expressions (MDN)

Class expressions are very similar to class declarations. The only real difference is that class expressions don’t need a name, though we can refer to them via whatever identifier they ended up bound to:

Constructor Signatures

JavaScript classes are instantiated with the new operator. Given the type of a class itself, the InstanceType utility type models this operation.

abstract Classes and Members

Classes, methods, and fields in TypeScript may be abstract .

An abstract method or abstract field is one that hasn’t had an implementation provided. These members must exist inside an abstract class , which cannot be directly instantiated.

The role of abstract classes is to serve as a base class for subclasses which do implement all the abstract members. When a class doesn’t have any abstract members, it is said to be concrete .

Let’s look at an example:

We can’t instantiate Base with new because it’s abstract. Instead, we need to make a derived class and implement the abstract members:

Notice that if we forget to implement the base class’s abstract members, we’ll get an error:

Abstract Construct Signatures

Sometimes you want to accept some class constructor function that produces an instance of a class which derives from some abstract class.

For example, you might want to write this code:

TypeScript is correctly telling you that you’re trying to instantiate an abstract class. After all, given the definition of greet , it’s perfectly legal to write this code, which would end up constructing an abstract class:

Instead, you want to write a function that accepts something with a construct signature:

Now TypeScript correctly tells you about which class constructor functions can be invoked - Derived can because it’s concrete, but Base cannot.

Relationships Between Classes

In most cases, classes in TypeScript are compared structurally, the same as other types.

For example, these two classes can be used in place of each other because they’re identical:

Similarly, subtype relationships between classes exist even if there’s no explicit inheritance:

This sounds straightforward, but there are a few cases that seem stranger than others.

Empty classes have no members. In a structural type system, a type with no members is generally a supertype of anything else. So if you write an empty class (don’t!), anything can be used in place of it:

Nightly Builds

How to use a nightly build of TypeScript

How JavaScript handles communicating across file boundaries.

The TypeScript docs are an open source project. Help us improve these pages by sending a Pull Request ❤

Last updated: May 08, 2024  

TypeScript & Class Constructor: A Complete Guide

Introduction.

TypeScript enriches JavaScript with types and classes, offering an organized approach to web development that can significantly improve code maintainability and reusability. In this guide, we delve deep into the functionality of TypeScript class constructors, unveiling their potential to streamline object-oriented programming.

Understanding TypeScript Classes

TypeScript, as a superset of JavaScript, provides a structure for building objects using classes. Classes in TypeScript are blueprints for creating objects, providing a clear specification of what each object should contain in terms of data and behaviors. A class contains variables (properties) to define data and functions (methods) to define actions. Here is a simple example of a TypeScript class:

This class defines a Vehicle with a constructor that initializes the make and model of the vehicle. The getInfo() method can then use these properties.

Constructors in TypeScript

Constructors are special methods within a class responsible for initializing new objects. Whenever a new instance is created, the constructor is called, and it’s where you can set up your object. Here’s how a basic constructor looks in TypeScript:

The constructor here takes one parameter and assigns it to the object’s ‘name’ property.

Parameter Properties in TypeScript

TypeScript offers an even more concise way to declare and initialize class members right in the constructor known as parameter properties. With parameter properties, you can get rid of the declaration of the property and the manual assignment within the constructor body:

This constructor syntax is doing the same thing as before, but with less code. The ‘public’ access modifier before ‘name’ implies that ‘name’ should be treated both as a constructor parameter and a class property.

Advanced Constructor Patterns

As we progress into more complex cases, TypeScript constructors can also include logic to handle inheritance, method overriding, and much more. For instance, here’s how subclassing and constructor inheritance look:

In the Bird class, we first call the constructor of the base class (Animal) using ‘super’, and then proceed to specify unique properties for the Bird class.

Using Access Modifiers

TypeScript introduces several keywords as access modifiers – ‘public’, ‘private’, and ‘protected’. When used in constructors, these can control the visibility and accessibility of class members:

The ‘id’ can only be accessed within the Robot class, ‘type’ can be accessed by subclasses, and ‘name’ is public and can be accessed freely.

Default Parameters and Overloading

Default parameters can be provided in constructors to set default values. TypeScript also supports method overloading, including constructors:

In the ColoredPoint class, default values for x, y, and color are provided, increasing versatility with the ability to omit arguments upon instantiation.

Putting It All Together: A Practical Example

With all these concepts in hand, let’s create a practical example involving multiple classes and constructors:

In this case, an Employee is a Person but with an additional salary property which can be modified using the raiseSalary method.

In conclusion, TypeScript’s typing system, coupled with class constructors, elevate JavaScript to a more robust level, one that simplifies the object creation process and adds a wealth of features beneficial for scalable projects. Understanding these concepts deepens your grasp on TypeScript and provides you the tools to craft more efficient, maintainable code.

Next Article: TypeScript: Object with Optional Properties

Previous Article: How to Properly Use '!' (Exclamation Mark) in TypeScript

Series: The First Steps to TypeScript

Related Articles

• TypeScript Function to Convert Date Time to Time Ago (2 examples)
• TypeScript: setInterval() and clearInterval() methods (3 examples)
• TypeScript sessionStorage: CRUD example
• Using setTimeout() method with TypeScript (practical examples)
• Working with window.navigator object in TypeScript
• TypeScript: Scrolling to a specific location
• How to resize the current window in TypeScript
• TypeScript: Checking if an element is a descendant of another element
• TypeScript: Get the first/last child node of an element
• TypeScript window.getComputerStyle() method (with examples)
• Using element.classList.toggle() method in TypeScript (with examples)
• TypeScript element.classList.remove() method (with examples)

Search tutorials, examples, and resources

• PHP programming
• Symfony & Doctrine
• Laravel & Eloquent
• Tailwind CSS
• Sequelize.js
• Mongoose.js

TypeScript Constructor Assignment: public and private Keywords

TypeScript includes a concise way to create and assign a class instance property from a constructor parameter.

Rather than:

One can use the private keyword instead:

The public keyword works in the same fashion, but also instructs the TypeScript compiler that it’s OK to access the property from outside the class.

Here’s a more complete example including the public keyword, as well as the result of not including a keyword:

Trending Tags

Popular Articles

• Typescript Unknown Vs Any (Dec 06, 2023)
• Typescript Undefined (Dec 06, 2023)
• Typescript Type Definition (Dec 06, 2023)
• Typescript Splice (Dec 06, 2023)
• Typescript Return Type Of Function (Dec 06, 2023)

TypeScript Constructor

Switch to English

Table of Contents

Introduction

Basic syntax of typescript constructor, parameter properties in typescript constructor, tips and tricks, common errors and how to avoid them.

• Ensure that you are not defining multiple constructors. TypeScript supports only one constructor per class.
• Use parameter properties for a more concise way of defining and initializing class members.
• Remember that constructors are automatically called when an object is created. So, make sure not to call them directly.
• Use the `new` keyword to create an object and trigger the constructor.

DEV Community

Posted on Apr 20, 2019 • Originally published at maksimivanov.com on Feb 22, 2019

Typescript Constructor Shorthand

Here is a thing, in Typescript there is a shorthand to create and assign class properties from constructor params.

Imagine you have following code, let’s say you have class User :

You can write same class using shorter syntax:

In this case Typescript will automatically generate thore properties. And yes both definitions will produce same Javascript code:

And it works not only for private access level modifier, you can use public or protected as well.

So you can use this constructor assignment technique to save some lines of code.

Top comments (6)

Templates let you quickly answer FAQs or store snippets for re-use.

• Joined May 6, 2019

This article was one of the top hits when I was searching for this concept, but I couldn't figure out the official term for this shorthand; turns out it's called "parameter properties", and the official documentation for them is at typescriptlang.org/docs/handbook/2... .

• Email [email protected]
• Location Charlottesville, VA
• Education BSCS WGU
• Work Between jobs
• Joined Sep 10, 2022

Exactly what brought me here: trying to figure out the name of this feature. Kudos for using language I thought to search for.

• Location SE Pennsylvania
• Work lead dev by day, head writer at CubicleBuddha.com in the night
• Joined Apr 7, 2019

Daaaaang. You know, I generally tend to stay away from classes because I prefer to separate data from logic (you know, “functional programming” style), but sometimes it’s really convenient to use a class for encapsulating a set of shared functions (like a date class). So this is really helpful. Thank you! :)

• Location Stockholm, Sweden
• Work Frontend Developer at Mojang
• Joined Nov 21, 2017

Glad to be helpful :)

• Joined Aug 24, 2020

Hi, i wonder if you can add exempel shorthand for interface with readonly property. I am having problem. interface abcd { readonly x: number; y: string; } class xyz extends abcd { constructor (readonly x, y:string=''){} } **** when i declare new xyz() says expect 2 parameters, shorthand does not work in this case.

• Location Vietnam
• Joined Jul 18, 2021

Thanks for a concise explanation!

Are you sure you want to hide this comment? It will become hidden in your post, but will still be visible via the comment's permalink .

Hide child comments as well

For further actions, you may consider blocking this person and/or reporting abuse

A common TypeScript error with useRef

Arc - Apr 15

Encore for TypeScript - Open Source Backend SDK & Rust runtime for Node.js

Marcus Kohlberg - May 6

Reducing the Boilerplate for Services Utilizing Subjects in Angular

Volodymyr Yepishev - May 7

SOLID - The Simple Way To Understand

Kevin Toshihiro Uehara - May 1

We're a place where coders share, stay up-to-date and grow their careers.

Advisory boards aren’t only for executives. Join the LogRocket Content Advisory Board today →

• Product Management
• Solve User-Reported Issues
• Find Issues Faster
• Optimize Conversion and Adoption
• Start Monitoring for Free

TypeScript, abstract classes, and constructors

TypeScript has the ability to define classes as abstract. This means they cannot be instantiated directly; only nonabstract subclasses can be. Let’s take a look at what this means when it comes to constructor usage.

Making a scratchpad

To dig into this, let’s create a scratchpad project to work with. We’re going to create a Node.js project and install TypeScript as a dependency.

We now have a package.json file set up. We need to initialize a TypeScript project as well:

This will give us a tsconfig.json file that will drive configuration of TypeScript. By default, TypeScript transpiles to an older version of JavaScript that predates classes. So we’ll update the config to target a newer version of the language that does include them:

Let’s create a TypeScript file called index.ts . The name is not significant; we just need a file to develop in.

Finally we’ll add a script to our package.json that compiles our TypeScript to JavaScript and then runs the JS with node:

Making an abstract class in TypeScript

Now we’re ready. Let’s add an abstract class with a constructor to our index.ts file:

Consider the ViewModel class above. Let’s say we’re building some kind of CRUD app; we’ll have different views. Each view will have a corresponding viewmodel , which is a subclass of the ViewModel abstract class.

The ViewModel class has a mandatory id parameter in the constructor. This is to ensure that every viewmodel has an id value. If this were a real app, id would likely be the value with which an entity was looked up in some kind of database.

Importantly, all subclasses of ViewModel should either:

• Not implement a constructor at all, leaving the base class constructor to become the default constructor of the subclass
• Implement their own constructor that invokes the ViewModel base class constructor

Taking our abstract class for a spin

Now let’s see what we can do with our abstract class.

First of all, can we instantiate our abstract class? We shouldn’t be able to do this:

Sure enough, running npm start results in the following error (which is also being reported by our editor, VS Code).

Tremendous. However, it’s worth remembering that abstract is a TypeScript concept. When we compile our TS, although it’s throwing a compilation error, it still transpiles an index.js file that looks like this:

As we can see, there’s no mention of abstract ; it’s just a straightforward class . In fact, if we directly execute the file with node index.js we can see an output of:

So the transpiled code is valid JavaScript even if the source code isn’t valid TypeScript. This all reminds us that abstract is a TypeScript construct.

Subclassing without a new constructor

Let’s now create our first subclass of ViewModel and attempt to instantiate it:

As the TypeScript compiler tells us, the second of these instantiations is legitimate because it relies on the constructor from the base class. The first is not because there is no parameterless constructor.

Subclassing with a new constructor

Having done that, let’s try subclassing and implementing a new constructor that has two parameters (to differentiate from the constructor we’re overriding):

Again, only one of the attempted instantiations is legitimate. viewModel3 is not because there is no parameterless constructor. viewModel4 is not because we have overridden the base class constructor with our new one, which has two parameters. Hence, viewModel5 is our “Goldilocks” instantiation — it’s just right!

It’s also worth noting that we’re calling super in the NewConstructorViewModel constructor. This invokes the constructor of the ViewModel base (or “super”) class. TypeScript enforces that we pass the appropriate arguments (in our case a single string ).

Over 200k developers use LogRocket to create better digital experiences

We’ve seen that TypeScript ensures correct usage of constructors when we have an abstract class. Importantly, all subclasses of abstract classes either:

• Do not implement a constructor at all, leaving the base class constructor (the abstract constructor) to become the default constructor of the subclass
• Implement their own constructor, which invokes the base (or “super”) class constructor with the correct arguments

LogRocket : Full visibility into your web and mobile apps

LogRocket is a frontend application monitoring solution that lets you replay problems as if they happened in your own browser. Instead of guessing why errors happen or asking users for screenshots and log dumps, LogRocket lets you replay the session to quickly understand what went wrong. It works perfectly with any app, regardless of framework, and has plugins to log additional context from Redux, Vuex, and @ngrx/store.

In addition to logging Redux actions and state, LogRocket records console logs, JavaScript errors, stacktraces, network requests/responses with headers + bodies, browser metadata, and custom logs. It also instruments the DOM to record the HTML and CSS on the page, recreating pixel-perfect videos of even the most complex single-page and mobile apps.

Try it for free .

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Reddit (Opens in new window)
• Click to share on LinkedIn (Opens in new window)
• Click to share on Facebook (Opens in new window)
• #typescript

Stop guessing about your digital experience with LogRocket

Recent posts:.

Creating JavaScript tables using Tabulator

Explore React Tabulator to create interactive JavaScript tables, easily integrating pagination, search functionality, and bulk data submission.

How to create heatmaps in JavaScript: The Heat.js library

This tutorial will explore the application of heatmaps in JavaScript projects, focusing on how to use the Heat.js library to generate them.

Eleventy adoption guide: Overview, examples, and alternatives

Eleventy (11ty) is a compelling solution for developers seeking a straightforward, performance-oriented approach to static site generation.

6 CSS tools for more efficient and flexible CSS handling

Explore some CSS tools that offer the perfect blend of efficiency and flexibility when handling CSS, such as styled-components and Emotion.

One Reply to "TypeScript, abstract classes, and constructors"

Could we clarify the motivation for this over interfaces (or types)? I suppose having the option of running code in the constructor would be one benefit.

Leave a Reply Cancel reply

How To Use The TypeScript Constructor Efficiently

Diving into TypeScript, the constructor plays a pivotal role in object-oriented programming. In this article, we'll break down its intricacies, offer best practices, and highlight common mistakes. Perfect for developers looking to sharpen their TypeScript skills.

💡 KEY INSIGHTS

• TypeScript constructors are special methods in a class for creating and initializing objects of that class, often with type-checked parameters.
• Using parameter properties in TypeScript constructors can simplify code by allowing property declaration and initialization in one place.
• Overload constructors in TypeScript enable multiple ways of initializing an object, each with different parameters or types.
• TypeScript's constructor allows for the enforcement of class invariants , ensuring that an object's state is valid right from its creation.

TypeScript has become a cornerstone for many modern JavaScript projects, bringing type safety and enhanced tooling. One fundamental concept within TypeScript is the constructor function. This article offers insights and best practices to harness its full potential in your projects.

The Basics Of TypeScript Constructor

Key components and parameters of a constructor, creating and using multiple constructors, best practices for typescript constructor implementation, common mistakes and how to avoid them, frequently asked questions, constructor syntax and usage, using the constructor parameters, multiple constructors in typescript.

Constructors in TypeScript are declared with the keyword constructor . This method typically contains initialization code. The basic syntax is:

For a practical example, consider a class Person . This class might have a constructor to initialize its properties, like name and age:

In TypeScript, you can simplify the parameter-to-property assignment using access modifiers directly in the constructor:

Technically, TypeScript does not support multiple constructors, but you can achieve something similar using optional parameters and overloads:

Access Modifiers In Constructors

Parameter properties, optional and default parameters, rest parameters.

To grasp TypeScript constructors more deeply, understanding their key components and parameters is crucial. These components play pivotal roles in initializing and setting up class objects effectively.

Access modifiers dictate the accessibility of the properties, methods, and constructors of a class. TypeScript offers three primary access modifiers: public , private , and protected .

In TypeScript, parameter properties let you create and initialize a class property in one place. This is a shortcut to eliminate redundancy.

Constructors can have optional parameters , allowing flexibility in object creation. Additionally, you can set default parameters that assign a value if none is provided.

Rest parameters allow you to represent an indefinite number of arguments as an array. It can be particularly handy when the exact number of parameters is unpredictable.

Overloading Constructors

Optional parameters and constructors, working with different data types.

While TypeScript does not support traditional multiple constructors like some other languages, there are ways to simulate this behavior to achieve desired results and flexibility in class instantiation.

Overloading lets you have multiple ways to initialize a class object. However, in TypeScript, overloading works differently. You define multiple signatures for the constructor, but only one implementation.

You can provide flexibility in your constructors by using optional parameters . These parameters are not mandatory during object creation.

By employing union types, you can allow a constructor parameter to accept multiple data types.

Avoiding Lengthy Constructors

Using parameter properties, handling default values, ensuring single responsibility.

Constructors are a foundational element in object-oriented programming. In TypeScript, using them correctly can not only streamline your code but also avoid potential pitfalls. Let's delve into some best practices to optimize your TypeScript constructors .

A constructor that's too long can be challenging to read and maintain. As a rule of thumb, if a constructor has more than three parameters, consider using an options object .

Parameter properties let you create and initialize class members in one place. This can significantly reduce boilerplate code.

To provide default values for constructor parameters, use the = syntax. This ensures that the parameter has a value if none is provided during object creation.

A constructor's primary role is to initialize the object. Avoid embedding complex logic or operations inside it. If initialization requires more steps, consider employing factory methods .

Overloading Without Signatures

Not assigning properties, missing super calls, using constructors for logic, ignoring optional parameters.

While TypeScript offers a range of features for object-oriented design, developers can sometimes make common mistakes when working with constructors. Addressing these mistakes is key to robust and maintainable code.

Constructor overloading allows multiple constructor functions in a class. However, TypeScript does not directly support it, making it necessary to use signature overloads .

It's easy to declare properties but forget to assign values in the constructor. This often leads to undefined errors at runtime.

When extending a class, it's crucial to call the super() method in the derived class constructor. Missing it can lead to errors and unexpected behaviors.

Constructors should primarily initialize properties. Avoid including complex business logic or external calls.

Overlooking optional parameters in TypeScript can lead to unwanted results. Always ensure optional parameters have a default or fallback.

Can I Overload Constructors in TypeScript Like in Other Languages?

Direct overloading, as seen in languages like Java, isn't supported. However, TypeScript allows constructor signature overloads. While you can have multiple signatures for a constructor, there will only be one implementation.

Why Do I Get an Error When I Don't Call super() in a Derived Class?

When a class extends another class, the constructor of the derived class must call super() . It ensures the correct initialization of the base class portion of the object.

How Do Optional Parameters in Constructors Work?

Optional parameters in TypeScript are denoted by a ? . When an argument for such a parameter isn't provided, its value becomes undefined . To ensure no issues, always provide a default or fallback value for optional parameters.

Is It Possible to Create a Private Constructor in TypeScript?

Yes, TypeScript supports private constructors. A class with a private constructor cannot be instantiated or extended outside of its containing class. It's often used in the Singleton pattern.

Let’s test your knowledge!

What's the primary role of a constructor in TypeScript?

Continue learning with these typescript guides.

• Exploring The Differences: Typescript Namespace Vs Module
• Exploring TypeScript Record Vs Object And Its Applications
• Exploring The Differences: Typescript Enum Vs Type
• TypeScript's == Vs ===: A Closer Look At Their Functions
• Navigating Angular Vs TypeScript And Its Benefits

Subscribe to our newsletter

Subscribe to be notified of new content on marketsplash..

Get 42% off the TypeScript bundle

See the bundle then add to cart and your discount is applied.

Write TypeScript like a pro.

Follow the ultimate TypeScript roadmap.

Ultimate Courses

TypeScript Classes and Constructors

by Todd Motto

Mar 7, 2023

3 mins read

Learn TypeScript the right way.

The most complete guide to learning TypeScript ever built. Trusted by 82,951 students .

with Todd Motto

In this post you’ll learn about TypeScript constructors, how to create classes, and how they differ to their ES5 counterpart of traditional constructor functions and prototype methods.

First, what is a class? A class is a special and self-contained segment of code that constructs brand new objects when created. A class in TypeScript can also include properties, methods, and constructor logic.

Second, what is a constructor? A constructor is a special function that exists inside a class, that is called only once when the object is created.

Classes are unique, and therefore so are the constructor function calls.

Classes and Constructors

Classes are syntax sugar for creating traditional constructor functions prototypes. They are cleaner, shorter, and much easier to read.

You may be familiar with things like .prototype in JavaScript which allow us to inherit properties from other objects.

In TypeScript, we can create a class definition like this:

This creates a class definition and defines a constructor , giving us a unique reference called Product to refer to when creating new objects.

The ES5 equivalent to a class like this would be this:

From the above, they look very similar, however the difference lies in the fact our class is not the constructor, but holds a constructor function. In our second example, the function Product() {} acts as both the class and constructor.

To pass data to your constructor function, we create a new instance of the class:

With our ES5 approach, things look very similar:

Using the class keyword helps create a specific place to write logic for your object initialization, where we assign any variables passed in through the constructor to the internal class.

Remember, everytime we use the new keyword we create a unique instance, so no two references are the same.

So that is how to use constructors with TypeScript, and how they differ from traditional ES5 classes in JavaScript - so what about properties and methods?

Well, we’ve already learned properties because when we assign this.name = name inside the constructor, we are creating a public property that’s available throughout the class.

With methods, the new class syntax allows us to very nicely add logic, for example:

In the traditional constructor function example, it would look like this:

The way you use the constructor function versus a TypeScript class remains the same, but I think at this point we can agree a class is much clearer to group and contain related logic.

Despite a TypeScript class being syntax sugar, it will still be compiled down to the ES5 JavaScript equivalent, unless you tell the TypeScript compiler not to via tsconfig.json - which depending on the browser support for classes may or may not be a good idea.

Methods that we create on TypeScript classes are also known as ‘instance methods’, as we create an instance (brand new object) every time.

What’s more, we can now safely check the object type of our instance via the instanceof keyword:

What’s also helpful, in terms of debugging, is visually being able to see your custom class object in the console when logging things out:

This shows you how a class and constructor function differs from a plain object literal in JavaScript.

🏆 Check out my series of TypeScript Courses where you’ll learn TypeScript in-depth with real-world scenarios and examples to supercharge your JavaScript development.

And there we have it! Your introduction to TypeScript classes, constructors, prototypes and methods and how they differ from ES5 constructor functions.

Related blogs 🚀

Readonly properties in typescript.

In this post you’ll learn how and when to use the readonly keyword for in TypeScript.

Mar 9, 2023

Abstract Classes in TypeScript

In this post you’ll learn how to create and use an abstract class in TypeScript.

Mar 6, 2023

Public vs Private Properties and Methods in TypeScript

In this post you’ll learn how to create both public and private members in TypeScript classes, which we can use for both properties and methods.

Mar 5, 2023

Strict Property Initialization in TypeScript

In later versions of TypeScript there’s the concept of a Strict Class Property Initializer, or Definite Assignment Assertion.

Feb 28, 2023

Static Properties and Methods in TypeScript

In this post you’ll learn about the static keyword in TypeScript classes so you can confidently use them on properties and methods.

Feb 27, 2023

Readonly Mapped Type in TypeScript

In this post you’ll learn how and when to use the Readonly mapped type TypeScript.

Feb 26, 2023

Free eBooks:

You've got mail! Enjoy.

Cookies are used to analyze traffic and optimize experience.

A newer version of this site just became available. Please refresh this page to activate it.

Amit Merchant Verified ($10/year for the domain)

A blog on PHP, JavaScript, and more

Constructor assignment of field variables in TypeScript

June 15, 2021 · TypeScript

When you work with classes with constructors in TypeScript, the usual way to declare the field variables is like following.

As you can tell, using this approach you will have to declare class field variables at three different places.

• In the property declaration.
• In the constructor parameters.
• In the property assignment in the constructor body.

This is pretty cumbersome if you ask me. What if I tell you there’s a better way to do this?

Enter Constructor Assignment.

Constructor Assignment

Using constructor assignment, you declare the field variables inline as the constructor parameters. We can rewrite the previous example using constructor assignment like so.

As you can tell, using this approach, you don’t have to write a lot of boilerplate code and things look tidy and concise!

» Share: Twitter , Facebook , Hacker News

Like this article? Consider leaving a

👋 Hi there! I'm Amit . I write articles about all things web development. You can become a sponsor on my blog to help me continue my writing journey and get your brand in front of thousands of eyes.

More on similar topics

Auto implemented properties in TypeScript

Avoid relative import paths in TypeScript

Type annotate arguments for subset of a given type in TypeScript

Awesome Sponsors

Download my eBook

Recommendation(s)

Get the latest articles delivered right to your inbox!

No spam guaranteed.

Follow me everywhere

More in "TypeScript"

Difference between Export and Export Default in TypeScript

Recently Published

Return HTTP response as a collection (Effectively) in Laravel NEW

Property hooks are coming in PHP 8.4

A free alternative to GitHub Copilot

Talk to websites and PDFs with this free Chrome Extension

RunJS — A JavaScript Playground on your desktop

Top Categories

• Web Application
• My Portfolio

The Most Useful GIT Commands

Get a FREE cheat sheet ( value of 8.99$ ) with the most useful GIT commands.

How To Add Multiple Constructors In TypeScript?

If you come from a C# background, you may want to add multiple constructors to a TypeScript class . Although TypeScript doesn't support multiple constructors, you can still achieve similar behavior.

In TypeScript, you can achieve a similar behavior  to adding multiple constructors by:

• Adding constructor overloads AND implementing a custom type guard.
• Using the static factory method to construct a class.
• Adding a partial class argument to the constructor .

This article explains those solutions with code examples.

Let's get to it 😎.

Here are some other  TypeScript tutorials  for you to enjoy:

• Create a global variable in TypeScript
• Create a queue in TypeScript
• How to use the yield keyword in TypeScript

Navigation Menu

Search code, repositories, users, issues, pull requests..., provide feedback.

We read every piece of feedback, and take your input very seriously.

Saved searches

Use saved searches to filter your results more quickly.

To see all available qualifiers, see our documentation .

• Notifications

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement . We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Param decorator in constructor assignment #30573

lsagetlethias commented Mar 24, 2019

• 👍 1 reaction

AlCalzone commented Mar 27, 2019

Sorry, something went wrong.

lsagetlethias commented Mar 27, 2019

No branches or pull requests