Reference Error: JavaScript

When any value is assigned to undeclared variable or assignment without the var keyword or variable is not in your current scope, it might lead to unexpected results and that’s why JavaScript presents a ReferenceError: assignment to undeclared variable 'x' in strict mode. And this error causes a problem in execution of functions.

Saif Sadiq

December 28, 2025

How does it feel when you go to give a job interview and after reaching the interview location you find out that the company for which you are here doesn’t even exist.

Obviously you got angry and your mind will start throwing negative thoughts.

Exactly same happens with JavaScript too.

When any value is assigned to undeclared variable or assignment without the var keyword or variable is not in your current scope, it might lead to unexpected results and that’s why JavaScript presents a ReferenceError: assignment to undeclared variable "x" in strict mode. And this error causes a problem in execution of functions.

If you’ve begun to try out JavaScript you might have encountered some pretty baffling errors. I know I sure did…

ReferenceError: assignment to undeclared variable “x”

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

Code without ‘var’ keyword

function foo() { 
  'use strict'; 
  bar = true; //variable not declared
} 
foo();

What you get after executing above program?? An Error?? 🙁

How do you need to code 🙂

Insert ‘var’ in front of your variable and see your program running

function foo() {
  'use strict';
  var bar = true; //declared variable here
}
foo();

Likewise there are many scripting factors possible to generate reference error in javascript.

ReferenceError: "x" is not defined

ReferenceError: deprecated caller or arguments usage

ReferenceError: can't access lexical declaration`X' before initialization

ReferenceError: reference to undefined property "x"

ReferenceError: invalid assignment left-hand side

Author

Saif Sadiq

Blogs: 12

Saif Sadiq is a community contributor with 7+ years of experience working across product, growth, and developer-focused platforms. Currently Director of Product & Growth at Apptile, he leads product strategy and cross-functional execution for no-code mobile app tooling. Saif previously worked at TestMu AI, contributing to product and growth initiatives for a cloud-based cross-browser testing platform, and has been recognized as a most-viewed blogger and writer.