Basics of Ecmascript#

  1. Right Click -> Inspect element -> Open the console tab


A string of characters that start and end with ' or "


Values are the simplest components in JavaScript. - 1 is a value - true is a value - "hello" is a value - function() {} is a value


Variables can change

Assigning a value to a variable

var dogSentence = 'Dogs are the bane of my existence.'

Only use var once, then you can refer to it a dogSentence unless you want to reinitialise it


Take any number (0 to many) of values in parenthesis and perform an action

Below is a function of the string variable that replaces text.

dogSentence.replace('dogs', 'those blasted dogs')

Note that the variable dogSentence does not actually change because most JavaScript functions takes the value we give it and returns a new value, without modifying the value we passed in.

Writing functions#

Make use of the function keyword, parenthesis (), a space and then enclosing brackets {} The contents of the parenthesis should return something, if you want, else assigning a variable to the result of the function won’t work.

function makeMoreExciting(string) {
  return string + '!!!!'

Using the function#


Returns: "hello!!!!"

Built-in functions (Standard Library)#

So what functions are built into ecmascript? Answer is here

Third party libraries#

There are also a lot of third party libraries like: jQuery, ReactJs, underscore, angular…


Arrays are things you can keep multiple things in, like a list of things. An ‘element’ is a single item.

var myCatFriends = ["bill", "tabby", "ceiling"]

Note: Arrays preserve ordering so an array will stay in a particular order if new elements are added or removed

Specifying an element:#

myCatFriends[0] returns 'bill'

Adding to the array#

myCatFriends.push('super hip cat')

Count how many elements in an array#



Arrays are good for lists, but if you want more rich data so asking for the address of bill an object is better for that. It is basically an association of keys and values

var firstCat = { name: "bill", lastName: "the cat", address: "The Alley" }

Note: objects do not preserve orgering and keys are not in quotes

You can create an array of objects:

var moodLog = [
    date: "10/20/2012",
    mood: "catnipped"
    date: "10/21/2012",
    mood: "nonplussed"

Or an object of arrays:

var favorites = {
  treats: ["bird sighting", "belly rub", "catnip"],
  napSpots: ["couch", "planter box", "human face"]


Callbacks are just functions that call other functions after some asynchronous task

Synchronous code like:

var photo = download('')
uploadPhotoTweet(photo, '@maxogden')

the second line / task cannot run until the first task has completed

If the download takes really long, all other ecmascript is blocked on the page until it completes. We should just want to block the second task.

Blocking execution should be avoided at all costs

If you want a task b() to run after a() has completed.

a(b), b is a callback to a

When we pass a callback function as an argument to another function, we are only passing the function definition. We are not executing the function in the parameter. Therefore we don’t add parenthesis () like we do when we are executing a function.

When we pass a callback function as an argument to another function, the callback is executed at some point inside the containing function’s body just as if the callback were defined in the containing function. So it is a closure, having access to the containing functions scope.

Callback Syntax#

saySomethingToSon('hello', saySon);

// Define main function, takes in url and name of callback method
function saySomethingToSon(phrase, callback) {
        // run our callback function

function saySon() {


Don’t ever use alert();

You should use console.log() instead as this works in both browser and server side. alert() works only on the server side

Sources: - Js for Cats - Essential JS Links