null vs undefined


  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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