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Showing content from https://developer.cdn.mozilla.net/en-US/docs/Web/JavaScript/Reference/Errors/Not_defined below:

ReferenceError: "x" is not defined - JavaScript

ReferenceError: "x" is not defined

The JavaScript exception "variable is not defined" occurs when there is a non-existent variable referenced somewhere.

ReferenceError: "x" is not defined (V8-based & Firefox)
ReferenceError: Can't find variable: x (Safari)

Error type

ReferenceError.

What went wrong?

There is a non-existent variable referenced somewhere. This variable needs to be declared, or you need to make sure it is available in your current script or scope.

Note: When loading a library (such as jQuery), make sure it is loaded before you access library variables, such as "$". Put the <script> element that loads the library before your code that uses it.

Examples Variable not declared

foo.substring(1); // ReferenceError: foo is not defined

The "foo" variable isn't defined anywhere. It needs to be some string, so that the String.prototype.substring() method will work.

const foo = "bar";
foo.substring(1); // "ar"

Wrong scope

A variable needs to be available in the current context of execution. Variables defined inside a function cannot be accessed from anywhere outside the function, because the variable is defined only in the scope of the function

function numbers() {
  const num1 = 2;
  const num2 = 3;
  return num1 + num2;
}

console.log(num1); // ReferenceError num1 is not defined.

However, a function can access all variables and functions defined inside the scope in which it is defined. In other words, a function defined in the global scope can access all variables defined in the global scope.

const num1 = 2;
const num2 = 3;

function numbers() {
  return num1 + num2;
}

console.log(numbers()); // 5