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delete obj.property vs obj.property = undefined

Written byPhuoc Nguyen
10 Jun, 2020
Assume that `obj` is an object, and `property` is the name of its property.


  1. Calling `obj.property = undefined` sets the value of property to `undefined`. The property is still there and appears if we iterate the properties of the object.
    let person = { name: 'John' };
    person.name = undefined;

    person.hasOwnProperty('name'); // true
    name in person; // true
    Object.keys(person); // ['name']
    for (let p in person) {
    } // 'name'
    `delete obj.property` will remove the property from the object. Let's revisit the sample code above, now with `delete person.name`:
    let person = { name: 'John' };
    delete person.name;

    person.hasOwnProperty('name'); // false
    name in person; // false
    Object.keys(person); // An empty array

    // Nothing is shown up in the Console
    for (let p in person) {
  2. `delete` can't delete an inherited property.
    const car = { branch: 'Audi' };

    const a4 = Object.create(car);
    console.log(a4.branch); // 'Audi'

    delete a4.branch;
    console.log(a4.branch); // 'Audi'
    In this case, we have to set the property to `undefined`:
    a4.branch = undefined;
    console.log(a4.branch); // undefined

Good to know

  1. `delete` doesn't work with array:
    const array = [1, 2, 3, 4, 5];

    delete array[1];
    console.log(array); // [1, empty, 3, 4, 5]
    If you want to remove an item from an array, use the `splice` method.
    const array = [1, 2, 3, 4, 5];
    array.splice(2, 1);
    console.log(array); // [1, 2, 4, 5]
    There's an alternative method to remove the last element from array:
    const array = [1, 2, 3, 4, 5];
    console.log(array); // [1, 2, 3, 4]
  2. We can use the ES6 spread operator to remove a property from an object:
    const { name, ...rest } = { name: 'Foo', age: 20 };

    console.log(name); // 'Foo'
    console.log(rest); // { age: '20' }
    It's also possible to remove a dynamic property:
    const property = 'name';
    const { [property]: value, ...rest } = { name: 'Foo', age: 20 };

    console.log(value); // 'Foo'
    console.log(rest); // { age: '20' }

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Phước Nguyễn