What is the difference between a JavaScript variable that is: null, undefined or undeclared?

Answer

In JavaScript, null, undefined, and undeclared variables represent different concepts of "no value" or "missing value." Understanding their differences is crucial to avoiding errors and writing cleaner code.

1. null

• Definition: null is an explicit value that represents the intentional absence of any object or value.
• Type: It is a primitive value but, due to a historical bug in JavaScript, typeof null returns "object".
• Usage: It is explicitly assigned by developers to indicate that a variable or object property does not currently have a value but is intended to have one later.

Example:

let a = null; // a is explicitly set to "no value"
console.log(a); // null
console.log(typeof a); // "object"

2. undefined

• Definition: undefined means a variable has been declared but has not been assigned a value yet.
• Type: It is a primitive value, and typeof undefined returns "undefined".
• Usage:
  • Automatically assigned to uninitialized variables.
  • A function returns undefined if it does not have a return statement.
  • Accessing an object property or array element that does not exist returns undefined.

Example:

let b; // b is declared but not initialized
console.log(b); // undefined
console.log(typeof b); // "undefined"

let obj = {};
console.log(obj.value); // undefined (property does not exist)

function noReturn() {}
console.log(noReturn()); // undefined

3. Undeclared

• Definition: An undeclared variable is one that has not been declared in the current scope. Attempting to use it will result in a ReferenceError.
• Behavior: Unlike null and undefined, undeclared variables are not part of the execution context.
• Usage: This usually happens when trying to use a variable without declaring it using var, let, or const. Example:

console.log(c); // ReferenceError: c is not defined

Note: If you accidentally assign a value to an undeclared variable (without strict mode), JavaScript creates it as a global variable.

Skopiuj kod
"use strict";
x = 5; // ReferenceError: x is not defined

Key Notes

• null is intentional, undefined is unintentional, and undeclared is an outright error.
• Always declare variables using let, const, or var to avoid undeclared errors.
• Use null when you want to explicitly clear a value.
• Avoid comparing values directly with == since it performs type coercion. Use === for strict equality checks.

console.log(null == undefined); // true (type coercion)
console.log(null === undefined); // false (strict check)

MDN Web Docs: null

MDN Web Docs: undefined