What is the difference between null and undefined in JavaScript?

What is the difference between null and undefined in JavaScript?

undefined means a variable has been declared but has not yet been assigned a value :



var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefined



null is an assignment value. It can be assigned to a variable as a representation of no value :



var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows object



From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

Proof :



console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)



and

null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'

Stack Overflow Q#5076944, score: 1663