JavaScript Objects

Last updated on Wednesday, August 9th, 2017

What Are Objects?

Objects are containers for a collection of related variables (properties) and functions (methods). Some native browser objects that you might be familiar with include the document object and the window object.

The document object contains related methods for querying the DOM:

  • document.getElementById()
  • document.getElementsByTagName()
  • document.querySelector()
  • document.querySelectorAll()
  • document.getElementsByClassName()

The window object contains properties and methods like:

  • document
  • alert()
  • parseFloat()
  • parseInt()

Properties and methods on window are part of the global scope, so you can either access them with window.*** such as window.document or window.alert(), or more concisely document and alert().

Object Literals

You can also create your own objects in a few different ways. One approach is to use the object literal syntax, which uses curly braces and key-value pairs separated by commas:

let cat = {
  name: 'Fiona',
  age: 1.5,
  siblings: ['Chestnut', 'Biscuit']
};

Object keys follow the same rules as variable definitions. If you want to use special characters or reserved words as keys, you can put the key in quotes:

let obj = {
  '$%#': 'some value'
};

Object properties can contain any value, so strings, numbers, booleans, arrays, functions, and other objects.

Properties can be accessed using dot notation or bracket notation:

let name = cat.name; // Fiona
name = cat['name']; // Fiona

Properties on an object can also be changed through dot or bracket notation:

cat.name = 'Chestnut';
cat['name'] = 'Fiona';

Object Methods

We can give objects custom behaviors / functions called methods. Inside a method, the object can be referenced using the this keyword.

let cat = {
  name: 'Fiona',
  meow: function() {
    console.log(this.name);
  }
};

cat.meow();

Inside the meow method, the name property on the cat object can be accessed by this.name. The value of this is determined when the function is invoked. When meow is called via cat.meow(), this corresponds to the object on which meow is called on, which in this case is the cat object.

Alternatively, you could write the meow method like this:

let cat = {
  name: 'Fiona',
  age: 1.5,
  siblings: ['Chestnut', 'Biscuit'],
  meow: function() {
    console.log(cat.name);
  }
};

We can explicitly reference the cat object within the meow method, but this has a drawback. If we rename the cat variable to something else, we would have to find every reference to cat within our object and replace it. Using the keyword this makes our methods less coupled to the name of the variable.

Methods can also call other methods:

let cat = {
  name: 'Fiona',
  age: 1.5,
  siblings: ['Chestnut', 'Biscuit'],
  meow: function(catWords) {
    console.log(catWords);
  },
  greet: function() {
    this.meow('Hi, my name is ' + this.name);
    this.meow('Give me food');
  }
};

cat.greet();

The cat greet() method calls the meow() method using the this keyword.

Before wrapping up, I want to mention that ES 2015 has introduced a new way to defining methods:

let cat = {
  name: 'Fiona',
  meow() {
    console.log(this.name);
  }
};

Notice how the : function part can be removed. Much more concise!

Disclaimer: Any viewpoints and opinions expressed in this article are those of David Tang and do not reflect those of my employer or any of my colleagues.

comments powered by Disqus