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