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null vs undefined

Written byPhuoc Nguyen
18 Jun, 2020


  1. `undefined` indicates that a variable has been declared but not been assigned to any value, including `null`.
    let foo;
    console.log(foo); // undefined
    `null` is used to indicate that a variable doesn't have a value.
    let foo = null;
    console.log(foo); // null
  2. `undefined` and `null` are considered to be different types.
    `undefined` is a type itself, whereas `null` is a special value of object.
    console.log(typeof undefined); // 'undefined'
    console.log(typeof null); // 'object'
    Since they are different types, here are the result of equality and identity operators when comparing them to each other:
    null == undefined; // true
    null === undefined; // false

Good to know

`JSON.stringify` omits `undefined`, but keeps `null`:
name: 'John',
address: null,
age: undefined,

// {"name":"John","address":null}

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Phước Nguyễn