Reference Error: JavaScript

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