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

ReferenceError in strict mode only.

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.

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

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

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();

• Strict mode