assignment to undeclared variable

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JavaScript ReferenceError – Assignment to undeclared variable

This JavaScript exception Assignment to undeclared variable occurs in strict-mode If the value has been assigned to an undeclared variable.

Error Type:

Cause of the error: Somewhere in the code, there is an assignment without the var , let, or const keywords. When a value is assigned to an undeclared variable this error occurs.

Example 1: In this example, the const keyword is used with the variable assignment, So the error has not occurred.

Output: 

Example 2: In this example, the var , let or const keyword is not used with the variable assignment, So the error has occurred.

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How to Fix the ‘ReferenceError: assignment to undeclared variable “x”‘ Error in Our JavaScript App?

• Post author By John Au-Yeung
• Post date August 20, 2021
• No Comments on How to Fix the ‘ReferenceError: assignment to undeclared variable “x”‘ Error in Our JavaScript App?

Sometimes, we may run into the ‘ReferenceError: assignment to undeclared variable "x"’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘ReferenceError: assignment to undeclared variable "x"’ when we’re developing JavaScript apps.

Fix the ‘ReferenceError: assignment to undeclared variable "x"’ When Developing JavaScript Apps

To fix the ‘ReferenceError: assignment to undeclared variable "x"’ when we’re developing JavaScript apps, we should make sure that we’re assigning values to variables that have already been declared.

The error message for this error is ReferenceError: "x" is not defined in Chrome.

And in Edge, the error message for this error is ReferenceError: Variable undefined in strict mode .

For instance, we shouldn’t write code like:

since bar hasn’t been declared in the foo function.

Instead, we should declare the bar variable by writing:

Now the error should be fixed.

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Web developer specializing in React, Vue, and front end development.

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ReferenceError: assignment to undeclared variable "x"

The JavaScript strict mode -only exception "Assignment to undeclared variable" occurs when the value has been assigned to an undeclared variable.

ReferenceError warning in strict mode only.

What went wrong?

A value has been assigned to an undeclared variable. In other words, there was an assignment without the var keyword. There are some differences between declared and undeclared variables, which might lead to unexpected results and that's why JavaScript presents an error in strict mode.

Three things to note about declared and undeclared variables:

• Declared variables are constrained in the execution context in which they are declared. Undeclared variables are always global.
• Declared variables are created before any code is executed. Undeclared variables do not exist until the code assigning to them is executed.
• Declared variables are a non-configurable property of their execution context (function or global). Undeclared variables are configurable (e.g. can be deleted).

For more details and examples, see the var reference page.

Errors about undeclared variable assignments occur in strict mode code only. In non-strict code, they are silently ignored.

Invalid cases

In this case, the variable "bar" is an undeclared variable.

Valid cases

To make "bar" a declared variable, you can add the var keyword in front of it.

• Strict mode

© 2005–2021 MDN contributors. Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Undeclared_var

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ReferenceError: assignment to undeclared variable "x"

The JavaScript strict mode -only exception "Assignment to undeclated variable" occurs when the value has been assigned to an undeclared variable.

ReferenceError warning in strict mode only.

What went wrong?

A value has been assigned to an undeclared variable. In other words, there was an assignment without the var keyword. There are some differences between declared and undeclared variables, which might lead to unexpected results and that's why JavaScript presents an error in strict mode.

Three things to note about declared and undeclared variables:

• Declared variables are constrained in the execution context in which they are declared. Undeclared variables are always global.
• Declared variables are created before any code is executed. Undeclared variables do not exist until the code assigning to them is executed.
• Declared variables are a non-configurable property of their execution context (function or global). Undeclared variables are configurable (e.g. can be deleted).

For more details and examples, see the var reference page.

Errors about undeclared variable assignments occur in strict mode code only. In non-strict code, they are silently ignored.

Invalid cases

In this case, the variable "bar" is an undeclared variable.

Valid cases

To make "bar" a declared variable, you can add the var keyword in front of it.

• Strict mode
• JavaScript basics
• JavaScript first steps
• JavaScript building blocks
• Introducing JavaScript objects
• Introduction
• Grammar and types
• Control flow and error handling
• Loops and iteration
• Expressions and operators
• Numbers and dates
• Text formatting
• Regular expressions
• Indexed collections
• Keyed collections
• Working with objects
• Details of the object model
• Using promises
• Iterators and generators
• Meta programming
• JavaScript modules
• Client-side JavaScript frameworks
• Client-side web APIs
• A re-introduction to JavaScript
• JavaScript data structures
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• Inheritance and the prototype chain
• JavaScript typed arrays
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• RangeError: radix must be an integer
• RangeError: repeat count must be less than infinity
• RangeError: repeat count must be non-negative
• ReferenceError: "x" is not defined
• ReferenceError: assignment to undeclared variable "x"
• ReferenceError: can't access lexical declaration "x" before initialization
• ReferenceError: deprecated caller or arguments usage
• ReferenceError: invalid assignment left-hand side
• ReferenceError: reference to undefined property "x"
• SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
• SyntaxError: "use strict" not allowed in function with non-simple parameters
• SyntaxError: "x" is a reserved identifier
• SyntaxError: JSON.parse: bad parsing
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• SyntaxError: for-in loop head declarations may not have initializers
• SyntaxError: function statement requires a name
• SyntaxError: identifier starts immediately after numeric literal
• SyntaxError: illegal character
• SyntaxError: invalid regular expression flag "x"
• SyntaxError: missing ) after argument list
• SyntaxError: missing ) after condition
• SyntaxError: missing : after property id
• SyntaxError: missing ; before statement
• SyntaxError: missing = in const declaration
• SyntaxError: missing ] after element list
• SyntaxError: missing formal parameter
• SyntaxError: missing name after . operator
• SyntaxError: missing variable name
• SyntaxError: missing } after function body
• SyntaxError: missing } after property list
• SyntaxError: redeclaration of formal parameter "x"
• SyntaxError: return not in function
• SyntaxError: test for equality (==) mistyped as assignment (=)?
• SyntaxError: unterminated string literal
• TypeError: "x" has no properties
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• TypeError: "x" is not a constructor
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• TypeError: Reduce of empty array with no initial value
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• TypeError: can't access property "x" of "y"
• TypeError: can't assign to property "x" on "y": not an object
• TypeError: can't define property "x": "obj" is not extensible
• TypeError: can't delete non-configurable array element
• TypeError: can't redefine non-configurable property "x"
• TypeError: cannot use "in" operator to search for "x" in "y"
• TypeError: cyclic object value
• TypeError: invalid "instanceof" operand "x"
• TypeError: invalid Array.prototype.sort argument
• TypeError: invalid arguments
• TypeError: invalid assignment to const "x"
• TypeError: property "x" is non-configurable and can't be deleted
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• TypeError: variable "x" redeclares argument
• URIError: malformed URI sequence
• Warning: -file- is being assigned a //# sourceMappingURL, but already has one
• Warning: 08/09 is not a legal ECMA-262 octal constant
• Warning: Date.prototype.toLocaleFormat is deprecated
• Warning: JavaScript 1.6's for-each-in loops are deprecated
• Warning: String.x is deprecated; use String.prototype.x instead
• Warning: expression closures are deprecated
• Warning: unreachable code after return statement
• JavaScript technologies overview
• Lexical grammar
• Enumerability and ownership of properties
• Iteration protocols
• Transitioning to strict mode
• Template literals
• Deprecated features

ReferenceError: assignment to undeclared variable "x"

The JavaScript strict mode -only exception "Assignment to undeclared variable" occurs when the value has been assigned to an undeclared variable.

ReferenceError in strict mode only.

What went wrong?

A value has been assigned to an undeclared variable. In other words, there was an assignment without the var keyword. There are some differences between declared and undeclared variables, which might lead to unexpected results and that's why JavaScript presents an error in strict mode.

Three things to note about declared and undeclared variables:

• Declared variables are constrained in the execution context in which they are declared. Undeclared variables are always global.
• Declared variables are created before any code is executed. Undeclared variables do not exist until the code assigning to them is executed.
• Declared variables are a non-configurable property of their execution context (function or global). Undeclared variables are configurable (e.g. can be deleted).

For more details and examples, see the var reference page.

Errors about undeclared variable assignments occur in strict mode code only. In non-strict code, they are silently ignored.

Invalid cases

In this case, the variable "bar" is an undeclared variable.

What went wrong?

ReferenceError warning in strict mode only.

A value has been assigned to an undeclared variable. In other words, there was an assignment without the var keyword. There are some differences between declared and undeclared variables, which might lead to unexpected results and that's why JavaScript presents an error in strict mode.

Three things to note about declared and undeclared variables:

• Declared variables are constrained in the execution context in which they are declared. Undeclared variables are always global.
• Declared variables are created before any code is executed. Undeclared variables do not exist until the code assigning to them is executed.
• Declared variables are a non-configurable property of their execution context (function or global). Undeclared variables are configurable (e.g. can be deleted).

For more details and examples, see the var reference page.

Errors about undeclared variable assignments occur in strict mode code only. In non-strict code, they are silently ignored.

Invalid cases

In this case, the variable "bar" is an undeclared variable.

Valid cases

To make "bar" a declared variable, you can add the var keyword in front of it.

• Strict mode

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• ReferenceError
• Strict Mode
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• JavaScript building blocks
• Introducing JavaScript objects
• Introduction
• Grammar and types
• Control flow and error handling
• Loops and iteration
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• Numbers and dates
• Text formatting
• Regular expressions
• Indexed collections
• Keyed collections
• Working with objects
• Details of the object model
• Using promises
• Iterators and generators
• Meta programming
• Client-side web APIs
• A re-introduction to JavaScript
• JavaScript data structures
• Equality comparisons and sameness
• Inheritance and the prototype chain
• JavaScript typed arrays
• Memory Management
• Concurrency model and Event Loop
• References:
• ArrayBuffer
• AsyncFunction
• Float32Array
• Float64Array
• GeneratorFunction
• InternalError
• Intl.Collator
• Intl.DateTimeFormat
• Intl.ListFormat
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• SharedArrayBuffer
• SyntaxError
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• Uint32Array
• Uint8ClampedArray
• WebAssembly
• decodeURI()
• decodeURIComponent()
• encodeURI()
• encodeURIComponent()
• parseFloat()
• Arithmetic operators
• Array comprehensions
• Assignment operators
• Bitwise operators
• Comma operator
• Comparison operators
• Conditional (ternary) operator
• Destructuring assignment
• Expression closures
• Generator comprehensions
• Grouping operator
• Legacy generator function expression
• Logical operators
• Object initializer
• Operator precedence
• (currently at stage 1) allows the creation of chained function calls in a readable manner. Basically, the pipeline operator provides syntactic sugar on a function call with a single argument allowing you to write">Pipeline operator
• Property accessors
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• element loaded from a different domain for which you violated the same-origin policy.">Error: Permission denied to access property "x"
• InternalError: too much recursion
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• RangeError: precision is out of range
• RangeError: radix must be an integer
• RangeError: repeat count must be less than infinity
• RangeError: repeat count must be non-negative
• ReferenceError: "x" is not defined
• ReferenceError: can't access lexical declaration`X' before initialization
• ReferenceError: deprecated caller or arguments usage
• ReferenceError: invalid assignment left-hand side
• ReferenceError: reference to undefined property "x"
• SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
• SyntaxError: "use strict" not allowed in function with non-simple parameters
• SyntaxError: "x" is a reserved identifier
• SyntaxError: JSON.parse: bad parsing
• SyntaxError: Malformed formal parameter
• SyntaxError: Unexpected token
• SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
• SyntaxError: a declaration in the head of a for-of loop can't have an initializer
• SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
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• SyntaxError: function statement requires a name
• SyntaxError: identifier starts immediately after numeric literal
• SyntaxError: illegal character
• SyntaxError: invalid regular expression flag "x"
• SyntaxError: missing ) after argument list
• SyntaxError: missing ) after condition
• SyntaxError: missing : after property id
• SyntaxError: missing ; before statement
• SyntaxError: missing = in const declaration
• SyntaxError: missing ] after element list
• SyntaxError: missing formal parameter
• SyntaxError: missing name after . operator
• SyntaxError: missing variable name
• SyntaxError: missing } after function body
• SyntaxError: missing } after property list
• SyntaxError: redeclaration of formal parameter "x"
• SyntaxError: return not in function
• SyntaxError: test for equality (==) mistyped as assignment (=)?
• SyntaxError: unterminated string literal
• TypeError: "x" has no properties
• TypeError: "x" is (not) "y"
• TypeError: "x" is not a constructor
• TypeError: "x" is not a function
• TypeError: "x" is not a non-null object
• TypeError: "x" is read-only
• TypeError: 'x' is not iterable
• TypeError: More arguments needed
• TypeError: Reduce of empty array with no initial value
• TypeError: can't access dead object
• TypeError: can't access property "x" of "y"
• TypeError: can't define property "x": "obj" is not extensible
• TypeError: can't delete non-configurable array element
• TypeError: can't redefine non-configurable property "x"
• TypeError: cannot use 'in' operator to search for 'x' in 'y'
• TypeError: cyclic object value
• TypeError: invalid 'instanceof' operand 'x'
• TypeError: invalid Array.prototype.sort argument
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• Warning: Date.prototype.toLocaleFormat is deprecated
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ReferenceError: can't access lexical declaration`X' before initialization

Valid cases.

• ReferenceError

What went wrong?

A lexical variable was accessed before it was initialized. This happens within any block statements, when a let or const is accessed before it's initialization.

Invalid cases

In this case, the variable "foo" is redeclared in the block so isn't initialized.

To make "foo" change inside the if statement you can't redeclare inside the new block statement.

• Temporal dead zone and errors with let

Document Tags and Contributors

• JavaScript basics
• JavaScript first steps
• JavaScript building blocks
• Introducing JavaScript objects
• Introduction
• Grammar and types
• Control flow and error handling
• Loops and iteration
• Expressions and operators
• Numbers and dates
• Text formatting
• Regular expressions
• Indexed collections
• Keyed collections
• Working with objects
• Details of the object model
• Iterators and generators
• Meta programming
• A re-introduction to JavaScript
• JavaScript data structures
• Equality comparisons and sameness
• Inheritance and the prototype chain
• Strict mode
• JavaScript typed arrays
• Memory Management
• Concurrency model and Event Loop
• References:
• ArrayBuffer
• AsyncFunction
• Float32Array
• Float64Array
• GeneratorFunction
• InternalError
• Intl.Collator
• Intl.DateTimeFormat
• Intl.NumberFormat
• ParallelArray
• SIMD.Bool16x8
• SIMD.Bool32x4
• SIMD.Bool64x2
• SIMD.Bool8x16
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• Uint8ClampedArray
• WebAssembly
• decodeURI()
• decodeURIComponent()
• encodeURI()
• encodeURIComponent()
• parseFloat()
• Arithmetic operators
• Array comprehensions
• Assignment operators
• Bitwise operators
• Comma operator
• Comparison operators
• Conditional (ternary) Operator
• Destructuring assignment
• Expression closures
• Generator comprehensions
• Grouping operator
• Legacy generator function expression
• Logical Operators
• Object initializer
• Operator precedence
• Property accessors
• Spread syntax
• async function expression
• class expression
• delete operator
• function expression
• function* expression
• in operator
• new operator
• void operator
• Legacy generator function
• async function
• for each...in
• function declaration
• try...catch
• Arguments object
• Arrow functions
• Default parameters
• Method definitions
• Rest parameters
• constructor
• element loaded from a different domain for which you violated the same-origin policy.">Error: Permission denied to access property "x"
• InternalError: too much recursion
• RangeError: argument is not a valid code point
• RangeError: invalid array length
• RangeError: invalid date
• RangeError: precision is out of range
• RangeError: radix must be an integer
• RangeError: repeat count must be less than infinity
• RangeError: repeat count must be non-negative
• ReferenceError: "x" is not defined
• ReferenceError: assignment to undeclared variable "x"
• ReferenceError: can't access lexical declaration`X' before initialization
• ReferenceError: deprecated caller or arguments usage
• ReferenceError: invalid assignment left-hand side
• ReferenceError: reference to undefined property "x"
• SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
• SyntaxError: "use strict" not allowed in function with non-simple parameters
• SyntaxError: "x" is a reserved identifier
• SyntaxError: JSON.parse: bad parsing
• SyntaxError: Malformed formal parameter
• SyntaxError: Unexpected token
• SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
• SyntaxError: a declaration in the head of a for-of loop can't have an initializer
• SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
• SyntaxError: for-in loop head declarations may not have initializers
• SyntaxError: function statement requires a name
• SyntaxError: identifier starts immediately after numeric literal
• SyntaxError: illegal character
• SyntaxError: invalid regular expression flag "x"
• SyntaxError: missing ) after argument list
• SyntaxError: missing ) after condition
• SyntaxError: missing : after property id
• SyntaxError: missing ; before statement
• SyntaxError: missing = in const declaration
• SyntaxError: missing ] after element list
• SyntaxError: missing formal parameter
• SyntaxError: missing name after . operator
• SyntaxError: missing variable name
• SyntaxError: missing } after function body
• SyntaxError: missing } after property list
• SyntaxError: redeclaration of formal parameter "x"
• SyntaxError: return not in function
• SyntaxError: test for equality (==) mistyped as assignment (=)?
• SyntaxError: unterminated string literal
• TypeError: "x" has no properties
• TypeError: "x" is (not) "y"
• TypeError: "x" is not a constructor
• TypeError: "x" is not a function
• TypeError: "x" is not a non-null object
• TypeError: "x" is read-only
• TypeError: More arguments needed
• TypeError: can't access dead object
• TypeError: can't define property "x": "obj" is not extensible
• TypeError: can't delete non-configurable array element
• TypeError: can't redefine non-configurable property "x"
• TypeError: cyclic object value
• TypeError: invalid 'in' operand "x"
• TypeError: invalid Array.prototype.sort argument
• TypeError: invalid arguments
• TypeError: invalid assignment to const "x"
• TypeError: property "x" is non-configurable and can't be deleted
• TypeError: setting getter-only property "x"
• TypeError: variable "x" redeclares argument
• URIError: malformed URI sequence
• Warning: -file- is being assigned a //# sourceMappingURL, but already has one
• Warning: 08/09 is not a legal ECMA-262 octal constant
• Warning: Date.prototype.toLocaleFormat is deprecated
• Warning: JavaScript 1.6's for-each-in loops are deprecated
• Warning: String.x is deprecated; use String.prototype.x instead
• Warning: expression closures are deprecated
• Warning: unreachable code after return statement
• JavaScript technologies overview
• Lexical grammar
• Enumerability and ownership of properties
• Iteration protocols
• Transitioning to strict mode
• Template literals
• Deprecated features
• ECMAScript 2015 support in Mozilla
• ECMAScript 5 support in Mozilla
• ECMAScript Next support in Mozilla
• Firefox JavaScript changelog
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