Objects are a data type that let us store a collection of properties and methods.

var objectName = {
  propertyName: propertyValue,
  propertyName: propertyValue,
  ...
};

var aboutMe = {
  hometown: "Pasadena, CA",
  hair: "brown"
};

var lizzieTheCat = {
  age: 18,
  furColor: "grey",
  likes: ["catnip", "milk"],
  birthday: {"month": 7, "day": 17, year: 1994}
};

Javascript has a more complex data type called an 'object'. Objects allow us to describe an entity that has a complex structure, and more closely resembles something that exist in the real world.
Arrays and functions are both special types of objects in Javascript. We can also define our own objects to represent complex data structures.
Just like objects in the real world, Javascript objects have attributes - called 'properties', and things they can do - called 'methods'. For example, we can create an object that represents a dog in Javascript. It could have properties like 'colour' and 'breed', and methods like 'bark' and 'chaseCat'.

Access properties using "dot notation":

var aboutMe = {
  hometown: "Pasadena, CA",
  hair: "brown"
};

var myHometown = aboutMe.hometown;

Or using "bracket notation" (like arrays):

var myHair = aboutMe["hair"];

Non-existent properties will return undefined:

var myGender = aboutMe["gender"];

Properties can also be accessed with variable keys

var myProperty = "hair";
var myHair = aboutMe[myProperty];

Use dot or bracket notation with the assignment operator to change objects.

Change existing properties:

var aboutMe = {
  hometown: "Pasadena, CA",
  hair: "brown"
};
aboutMe.hair = "blue";

Or add new properties:

aboutMe.gender = "female";

You can also delete properties:

delete aboutMe.gender;

Since arrays can hold any data type, they can also hold objects:

var myCats = [
  {name: "Lizzie",
   age: 18},
  {name: "Daemon",
   age: 1}
];

for (var i = 0; i < myCats.length; i++) {
  var myCat = myCats[i];
  console.log(myCat.name + ' is ' + myCat.age + ' years old.');
}

Just like other data types, objects can be passed into functions:

var lizzieTheCat = {
  age: 18,
  furColor: "grey",
  likes: ["catnip", "milk"],
  birthday: {"month": 7, "day": 17, year: 1994}
}

function describeCat(cat) {
  console.log("This cat is " + cat.age + " years old with " + cat.furColor + " fur.");
}

describeCat(lizzieTheCat);

Object properties can also be functions. Object functions are called "methods".

var lizzieTheCat = {
  age: 18,
  furColor: 'grey',
  meow: function() {
    console.log('meowww');
  },
  eat: function(food) {
    console.log('Yum, I love ' + food);
  },
  sleep: function(numMinutes) {
    for (var i = 0; i < numMinutes; i++) {
      console.log('z');
    }
  }
};

Call object methods using dot notation:

lizzieTheCat.meow();
lizzieTheCat.eat('brown mushy stuff');
lizzieTheCat.sleep(10);

Object.keys() lists all the property names of an object in an array.

var lizzieTheCat = {
  age: 18,
  furColor: 'grey',
  meow: function() {
    console.log('meowww');
  },
  eat: function(food) {
    console.log('Yum, I love ' + food);
  },
  sleep: function(numMinutes) {
    for (var i = 0; i < numMinutes; i++) {
      console.log('z');
    }
  }
};

Object.keys(lizzieTheCat); // ["age", "furColor", "meow", "eat", "sleep"]

JS provides several built-in objects:

• Array
  
Array.isArray()
• Number
  
Number()
Number.parseInt()
Number.parseFloat()
• Date
  
Date.UTC()
Date.now()
Date.parse()
• Math
  Sooooooooo many useful things!