JavaScript Reference: Object

Anything that is not a primitive (string, number, boolean, symbol, null, or undefined) is an object. Arrays and functions are objects too.

Create an object

The object literal is the form you will use most:

const person = {}
typeof person //object

These are equivalent ways to make a plain object:

const person = Object()
const person = new Object()
const car = Object.create()

Initialize properties inline:

const person = {
  age: 36,
  name: 'Flavio',
  speak: () => {
    //speak
  }
}

const person = Object({
  age: 36,
  name: 'Flavio',
  speak: () => {
    //speak
  }
})

const person = new Object({
  age: 36,
  name: 'Flavio',
  speak: () => {
    //speak
  }
})

You can also use a constructor function:

function Car(brand, model) {
  this.brand = brand
  this.model = model
}

Then instantiate it:

const myCar = new Car('Ford', 'Fiesta')
myCar.brand //'Ford'
myCar.model //'Fiesta'

Properties, methods, and references

Objects store properties: a name and a value. Values can be any type, including nested objects.

When a property value is a function, we call it a method.

Objects can inherit properties from other objects. We cover that in prototypal inheritance.

Objects are passed by reference. Primitives copy by value:

let age = 36
let myAge = age
myAge = 37
age //36

Two variables can point at the same object:

const car = {
  color: 'blue'
}
const anotherCar = car
anotherCar.color = 'yellow'
car.color //'yellow'

Built-in Object properties

The Object constructor exposes two properties:

• length always equal to 1
• prototype this points to the Object prototype object: the object that all other objects inherit from. Check the prototypal inheritance post for more.

Static methods

Static methods live on Object itself. Instance methods run on a particular object.

Static helpers keep related utilities namespaced instead of polluting globals:

• Object.assign() *ES2015
• Object.create()
• Object.defineProperties()
• Object.defineProperty()
• Object.entries() *ES2017
• Object.freeze()
• Object.getOwnPropertyDescriptor()
• Object.getOwnPropertyDescriptors()
• Object.getOwnPropertyNames()
• Object.getOwnPropertySymbols()
• Object.getPrototypeOf()
• Object.is() *ES2015
• Object.isExtensible()
• Object.isFrozen()
• Object.isSealed()
• Object.keys()
• Object.preventExtensions()
• Object.seal()
• Object.setPrototypeOf() *ES2015
• Object.values()

Instance methods

Every plain object inherits these methods on its prototype:

• hasOwnProperty()
• isPrototypeOf()
• propertyIsEnumerable()
• toLocaleString()
• toString()
• valueOf()

Create { name: 'Flavio' }, copy it to a second variable, change name through the copy, and log the original. That one experiment shows reference sharing clearly.