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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"

Valid cases.

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.

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

• Strict mode

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Dec 28, 2016 8:00:56 AM | JavaScript Error Handling - ReferenceError: assignment to undeclared variable “x”

The Assignment to Undeclared Variable error crops up anytime code attempts to assign a value to a variable that has yet to be declared.

Today as we progress through our  JavaScript Error Handling  series we're going to take a closer look at the Assignment to Undeclared Variable error in all its magnificent glory. The Assignment to Undeclared Variable error crops up anytime code attempts to assign a value to a variable that has yet to be declared (via the var keyword).

Below we'll go over a few code examples that will illustrate reproduction of a typical Assignment to Undeclared Variableerror, as well as outline how to handle this error when it comes about. Let's get to it!

The Technical Rundown

• All JavaScript error objects are descendants of the  Error  object, or an inherited object therein.
• The  ReferenceError  object is inherited from the  Error  object.
• The Assignment to Undeclared Variable error is a specific type of  ReferenceError  object.

When Should You Use It?

As mentioned in the introduction, an Assignment to Undeclared Variable error will occur when attempting to assign a value to a variable which has not previously been declared using the var keyword. However, beyond this cause, there is also one strict limitation within JavaScript itself, which can cause Assignment to Undeclared Variable errors to be  suppressed  in many circumstances:  An Assignment to Undeclared Variable error will only fire when  Strict Mode  is active within the executing JavaScript.

When  Strict Mode  is activated, it is a way to programmatically opt in to a restricted variant of JavaScript that has intentionally different semantics and behaviors in a number of cases. While the entirety of Strict Mode is out of the scope of this article, for the purposes of analyzing errors like the Assignment to Undeclared Variable error, it's critical to understand that Strict Mode causes JavaScript to  disallow  the creation of accidental global variables.

For example, in normal JavaScript, the following code would be allowed, as the JavaScript engine assumes the intention is to create a new global variable, names, and assign it to the value of splitting the full name string of "Bob Smith" that was provided:

Sure enough, in normal JavaScript, this executes properly without any thrown errors and gives us the expected output:

Therefore, in order for JavaScript to assume it is not the intention of the developer and the code to automatically generate a new global variable anytime an assignment is made to a previously undeclared variable, Strict Mode must be enabled. This is done by including the line 'use strict'; somewhere in the code, preferably at the same scope as the checks for undeclared assignments.

In this example, we are expanding a bit on the concept above, by creating a simple getFirstName() function, which takes a single full_name parameter, enters into Strict Mode, splits the full_name, and returns the first value (i.e. the first name). By assigning Strict Mode within the function only, we ensure our strict behavior is only within the scope we want, rather than globally across our entire script:

var getFirstName = function(full_name) { // Setting strict mode 'use strict'; // Assigning to `names`, which is undefined names = full_name.split(' '); return names[0]; }

try { var first_name = getFirstName("Bob Smith"); console.log(`First name is ${first_name}.`) } catch (e) { if (e instanceof ReferenceError) { printError(e, true); } else { printError(e, false); } }

Now that Strict Mode is enabled, the line that worked previously and created a new names global variable now, instead, produces an Assignment to Undeclared Variable error as expected. The specific error message produced will differ slightly depending on the JavaScript engine, but the idea is the same:

// CHROME [EXPLICIT] ReferenceError: names is not defined

The obvious solution to our issue is to ensure that anytime a variable assignment is made, it must already be declared with proper scope,  or  we must always specify this new variable declaration with the var keyword preceding it:

var getFirstName = function(full_name) { // Setting strict mode 'use strict'; // Assigning to `names`, which is now defined var names = full_name.split(' '); return names[0]; }

With no Assignment to Undeclared Variable error produced, as expected, we get our first name output:

These built-in exception classes are helpful JavaScript error handling tools! Used with Airbrake’s JavaScript Error Handler your debugging process will be a breeze. Good luck!

Written By: Frances Banks

You may also like.

Jan 3, 2017 11:34:16 AM | JavaScript Error Handling - ReferenceError: "x" is not defined

Jan 26, 2017 6:00:03 am | javascript - referenceerror: invalid assignment left-hand side, may 13, 2017 8:00:02 am | javascript error handling - syntaxerror: redeclaration of formal parameter "x".

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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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By John Au-Yeung

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 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 a let , const , or var keyword in front of it.

• Strict mode

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.

Valid cases

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

• Strict mode

© 2005–2023 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