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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.

Message

ReferenceError: x is not defined (V8-based)
ReferenceError: assignment to undeclared variable x (Firefox)
ReferenceError: Can't find variable: x (Safari)

Error type

ReferenceError in strict mode only.

What went wrong?

You have an assignment of the form x = ..., but x has not been previously declared with the var, let, or const keyword. This error occurs in strict mode code only. In non-strict code, assignment to an undeclared variable implicitly creates a property on the global scope.

Examples

Invalid cases

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

function foo() {
  "use strict";
  bar = true;
}
foo(); // ReferenceError: assignment to undeclared variable bar

Valid cases

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

function foo() {
  "use strict";
  const bar = true;
}
foo();