Can you tell more about the differences between null and undefined in JavaScript?

• undefined:

  • Assigned to variables that have been declared but not initialized with a value.
  • It is the default value for function parameters that are not passed arguments.
  • Occurs when trying to access a non-existent property of an object or an array element.
  • Data type — undefined.

• null:

  • A special value explicitly assigned to a variable to indicate "absence of a value".
  • Represents the intentional absence of any object or primitive value.
  • Data type — object (this is a historical JavaScript error, but it is so).

Main differences in the table:

Characteristic	undefined	null
Origin	Implicit (by default)	Explicit (assigned by the developer)
Purpose	Declared but not assigned a value	Intentional absence of a value
Data type (typeof)	undefined	object
Boolean context	false	false
Equality (==)	null == undefined (true)	null == undefined (true)
Strict equality (===)	null === undefined (false)	null === undefined (false)

Example:

let a; // Declared but not initialized -> undefined
console.log(a); // undefined

const obj = {};
console.log(obj.prop); // Accessing a non-existent property -> undefined

function greet(name) {
  console.log(name);
}
greet(); // Parameter name not passed -> undefined

let b = null; // Explicitly assigned null
console.log(b); // null

let c = {};
c.data = null; // Property data assigned null
console.log(c.data); // null

console.log(typeof a); // undefined
console.log(typeof b); // object
console.log(null == undefined); // true
console.log(null === undefined); // false