[Javascript](EN) Difference between undefined and null

Difference between undefined and null


Environment and Prerequisite

  • Javascript


undefined and null

undefined

  • It is a primitive value used when a variable has not been assigned a value.
  • A function returns undefined if explicitly nothing is returned.
  • One of primitive types
// test default
let test;
console.log('variable test is ' + test);

// if statement
if (test === undefined) {
  console.log('variable test is ' + test);
}

// if statement
if (undefined) {
  console.log('undefined is considered as true');
} else {
  console.log('undefined is considered as false');
}

// type test in if statement
// return type of typeof function is string
if (typeof(test) === 'undefined') {
  console.log('variable test value is ' + test + ' and type is ' + typeof(test));
}

// function return
function testReturnNothing() {
  let c;
}
console.log('testReturnNothing returns ' + testReturnNothing());

// check which is not defined
if (typeof(notDefinedVariable) === 'undefined') {
  console.log('variable notDefinedVariable is not defined');
}

variable test is undefined
variable test is undefined
undefined is considered as false
variable test value is undefined and type is undefined
testReturnNothing returns undefined
variable notDefinedVariable is not defined


null

  • It is a primitive value that represents the intentional absence of any object value.
  • In APIs, null is often retrieved in a place where an object can be expected but no object is relevant.
  • Treated as falsy for boolean operations.
// test default
let test = null;
console.log('variable test is ' + test);

// if statement
if (test === null) {
  console.log('variable test is ' + test);
}

// if statement
if (null) {
  console.log('null is considered as true');
} else {
  console.log('null is considered as false');
}

// type test in if statement
// return type of typeof function is string
// its return type is not 'null' it is 'object'
if (typeof(test) === 'object') {
  console.log('variable test value is ' + test + ' and type is ' + typeof(test));
}

// function return
function testReturnNull() {
  return null;
}
console.log('testReturnNull returns ' + testReturnNull());

variable test is null
variable test is null
null is considered as false
variable test value is null and type is object
testReturnNull returns null


Comparison

undefined

  • Value which represents not defined yet

null

  • Intentional value that it is absence

etc

  • It is equal when use == operator but it will be not equal when use === operator.
  • Because === operator compare not only value but also type. Its type is different like below.
typeof(null)         // object
typeof(undefined)    // undefined

null === undefined   // false
null == undefined    // true

!null                // true
!undefined           // true


Reference

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