Skip to content

JavaScript Objects

Destructure 3 levels down

match = obj?.props?.match;

If you know the property will exist

const {
    props: { match },
} = obj;
const {
    a: {
        b: { c },
    },
} = obj;

Default Dict

(obj['key'] || obj['key'] = []).push('value')

(obj['key'] || obj['key'] = 0) += 1

new keyword

  1. Creates a new object 2. type: object
  2. It sets this new object's internal, inaccessible, [[prototype]] (proto) property to be the constructor function's external, accessible, prototype object (every function object automatically has a prototype property).
  3. Variable points to the newly created object.
  4. Executes the constructor function
  5. Return the new object
    1. unless the constructor function returns a non-null object reference.
    2. In this case, that object reference is returned instead.

How do you know if they used new?

function Foo() {
    if (!new.target) {
        throw "Foo() must be called with new";
    }
}

Why Set() returns an error without new

  • without the new, the constructor will get called as a regular function
  • it will use this from the caller's context and not from Set and it might break

Last update: 2022-11-04