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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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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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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- Destructuring assignment
Unpacking values from a regular expression match
Es2015 version, invalid javascript identifier as a property name.
The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.
Description
The object and array literal expressions provide an easy way to create ad hoc packages of data.
The destructuring assignment uses similar syntax, but on the left-hand side of the assignment to define what values to unpack from the sourced variable.
This capability is similar to features present in languages such as Perl and Python.
Array destructuring
Basic variable assignment, assignment separate from declaration.
A variable can be assigned its value via destructuring separate from the variable's declaration.
Default values
A variable can be assigned a default, in the case that the value unpacked from the array is undefined .
Swapping variables
Two variables values can be swapped in one destructuring expression.
Without destructuring assignment, swapping two values requires a temporary variable (or, in some low-level languages, the XOR-swap trick ).
Parsing an array returned from a function
It's always been possible to return an array from a function. Destructuring can make working with an array return value more concise.
In this example, f() returns the values [1, 2] as its output, which can be parsed in a single line with destructuring.
Ignoring some returned values
You can ignore return values that you're not interested in:
You can also ignore all returned values:
Assigning the rest of an array to a variable
When destructuring an array, you can unpack and assign the remaining part of it to a variable using the rest pattern:
Note that a SyntaxError will be thrown if a trailing comma is used on the left-hand side with a rest element:
When the regular expression exec() method finds a match, it returns an array containing first the entire matched portion of the string and then the portions of the string that matched each parenthesized group in the regular expression. Destructuring assignment allows you to unpack the parts out of this array easily, ignoring the full match if it is not needed.
Object destructuring
Basic assignment, assignment without declaration.
A variable can be assigned its value with destructuring separate from its declaration.
The ( .. ) around the assignment statement is required syntax when using object literal destructuring assignment without a declaration.
{a, b} = {a: 1, b: 2} is not valid stand-alone syntax, as the {a, b} on the left-hand side is considered a block and not an object literal.
However, ({a, b} = {a: 1, b: 2}) is valid, as is var {a, b} = {a: 1, b: 2}
NOTE: Your ( ..) expression needs to be preceded by a semicolon or it may be used to execute a function on the previous line.
Assigning to new variable names
A property can be unpacked from an object and assigned to a variable with a different name than the object property.
A variable can be assigned a default, in the case that the value unpacked from the object is undefined .
Setting a function parameter's default value
Es5 version, nested object and array destructuring, for of iteration and destructuring, unpacking fields from objects passed as function parameter.
This unpacks the id , displayName and firstName from the user object and prints them.
Computed object property names and destructuring
Computed property names, like on object literals , can be used with destructuring.
Rest in Object Destructuring
The Rest/Spread Properties for ECMAScript proposal (stage 3) adds the rest syntax to destructuring. Rest properties collect the remaining own enumerable property keys that are not already picked off by the destructuring pattern.
Destructuring can be used with property names that are not valid JavaScript identifiers by providing an alternative identifer that is valid.
Specifications
Browser compatibility.
[1] Requires "Enable experimental Javascript features" to be enabled under `about:flags`
Firefox-specific notes
- Firefox provided a non-standard language extension in JS1.7 for destructuring. This extension has been removed in Gecko 40 (Firefox 40 / Thunderbird 40 / SeaMonkey 2.37). See bug 1083498 .
- Starting with Gecko 41 (Firefox 41 / Thunderbird 41 / SeaMonkey 2.38) and to comply with the ES2015 specification, parenthesized destructuring patterns, like ([a, b]) = [1, 2] or ({a, b}) = { a: 1, b: 2 } , are now considered invalid and will throw a SyntaxError . See Jeff Walden's blog post and bug 1146136 for more details.
- Assignment operators
- "ES6 in Depth: Destructuring" on hacks.mozilla.org
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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- Generator comprehensions
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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
- Equality comparisons and sameness
- Inheritance and the prototype chain
- JavaScript typed arrays
- Memory Management
- Concurrency model and Event Loop
- References:
- AggregateError
- ArrayBuffer
- AsyncFunction
- BigInt64Array
- BigUint64Array
- FinalizationRegistry
- Float32Array
- Float64Array
- GeneratorFunction
- InternalError
- ReferenceError
- SharedArrayBuffer
- SyntaxError
- Uint16Array
- Uint32Array
- Uint8ClampedArray
- WebAssembly
- decodeURI()
- decodeURIComponent()
- encodeURI()
- encodeURIComponent()
- parseFloat()
- Arithmetic operators
- Assignment operators
- Bitwise operators
- Comma operator
- Comparison operators
- Conditional (ternary) operator
- Destructuring assignment
- Function expression
- Grouping operator
- Logical AND
- Logical NOT
- Logical operators
- Nullish coalescing operator
- Object initializer
- Operator precedence
- Optional chaining
- Pipeline operator
- Property accessors
- Spread syntax
- async function expression
- class expression
- delete operator
- function* expression
- in operator
- new operator
- void operator
- async function
- for await...of
- function declaration
- import.meta
- try...catch
- Arrow function expressions
- Default parameters
- Method definitions
- Rest parameters
- The arguments object
- Private class fields
- Public class fields
- constructor
- 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: "x" is not iterable
- TypeError: More arguments needed
- TypeError: Reduce of empty array with no initial value
- TypeError: X.prototype.y called on incompatible type
- TypeError: can't access dead object
- 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
- 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
ReferenceError: assignment to undeclared variable "x"
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
© 2016 Mozilla 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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ES Module type=module cause "parcelRequire is not defined" error #1401
tarepan commented May 20, 2018 • edited
Thank you good library, I hope this project become better and better.
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@DevinGRhode - on the one hand, I agree. On the other hand, I see only two possibilities: a) you wish to create an expression based on existing JS variable(s), in which case there is a JS context that defined that variable. The only reason OP had trouble is they were using "short" form of maps return value, that lacked the explicit {...}.The fix was to convert to "long" form.
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.
TypeError: invalid Array.prototype.sort argument. TypeError: invalid assignment to const "x". TypeError: More arguments needed. TypeError: property "x" is non-configurable and can't be deleted. TypeError: Reduce of empty array with no initial value. TypeError: setting getter-only property "x". TypeError: X.prototype.y called on incompatible type.
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. ... Web developer specializing in React, Vue, and front end ...
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.
This JavaScript exception Assignment to undeclared variable occurs in strict-mode If the value has been assigned to an undeclared variable. Message: ReferenceError: assignment to undeclared variable "x" (Firefox) ReferenceError: "x" is not defined (Chrome) ReferenceError: Variable undefined in strict mode (Edge)
ReferenceError: assignment to undeclared variable "x" ReferenceError: deprecated caller or arguments usage; ReferenceError: invalid assignment left-hand side; ReferenceError: reference to undefined property "x" SyntaxError: "use strict" not allowed in function with non-simple parameters; SyntaxError: "x" is not a legal ECMA-262 octal constant
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.
Assignment separate from declaration. A variable can be assigned its value via destructuring separate from the variable's declaration. var a, b; [a, b] = [1, 2]; console.log(a); // 1 console.log(b); // 2 Default values. A variable can be assigned a default, in the case that the value unpacked from the array is undefined.
Before you do that, verify that none of your dependencies uses this variable!To do so, run your app, open console and type window.type.If it outputs something different from undefined, find and get rid of the library that uses this variable or switch to another drawing library (for example, geoman).. This trick works for my build process, though, you might want to modify it for your build system.
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.
The catch binding variable. In assignment patterns, the pattern does not start with a keyword. Each destructured property is assigned to a target of assignment — which may either be declared beforehand with var or let, or is a property of another object — in general, anything that can appear on the left-hand side of an assignment expression.
It seems to be an issue with leaflet-draw rather than react-leaflet-draw, which is also an unmaintained package, although some people seem to have found a workaround for this particular bug here ().
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. Examples Invalid cases. In this case, the variable "bar" is an undeclared variable. function foo ...
RangeError: x can't be converted to BigInt because it isn't an integer. 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: reference to undefined property "x".
Using ES6 syntax in React does not bind this to user-defined functions however it will bind this to the component lifecycle methods.. So the function that you declared will not have the same context as the class and trying to access this will not give you what you are expecting.. For getting the context of class you have to bind the context of class to the function or use arrow functions.
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.
I have the same problem in different context. I'm trying to consume parcel-bundled module from my webpack-bundled app. Since ES6 modules (my app) run in strict context an attempt to consume the module results in assignment to undefined variable and crashes my app.
ReactJS: Uncaught ReferenceError: [variable] not defined. I am new to Reactjs and I am having an issue with undefined variable. In the console log I am getting "Uncaught ReferenceError: drawResults is not defined app.js:116002" from the following React page: import React, { Component } from "react"; import ReactDOM from "react-dom"; import ...