Javascript built-in data structure - null and undefined

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javascriptfundamentals

This tutorial is a part of the Learn everything about Javascript in one course.

Both null and undefined data type are used to represent an absence of value. The biggest difference is, if null is used, it's usually because the program writer specifies it.

null undefined
represent absence of value absence of value
function does not return anything
usually assigned by program writer intentionally program writer intentionally
system runtime
null-and-undefined.js
// both myVar1 and myVar2 has no value
let myVar1 = null
let myVar2
console.log(myVar1) // null, I intentionally put it as null
console.log(myVar2) // undefined, system default

const myFunction = () => { } // a empty function, returns nothing
console.log(myFunction()) // undefined, default returned value of function does not return anything

Summary

  • Both null and undefined represent an absence of value.
  • null is usually assigned by coder, undefined usually assigned by system.