# JS Language

This chapter will not give you a general introduction to JavaScript. There are other books out there for a general introduction to JavaScript, please visit this great side on Mozilla Developer Network (opens new window).

On the surface JavaScript is a very common language and does not differ a lot from other languages:

function countDown() {
  for(var i=0; i<10; i++) {
    console.log('index: ' + i)

function countDown2() {
  var i=10;
  while( i>0 ) {

But be warned JS has function scope and not block scope as in C++ (see Functions and function scope (opens new window)).

The statements if ... else, break, continue also work as expected. The switch case can also compare other types and not just integer values:

function getAge(name) {
  // switch over a string
  switch(name) {
  case "father":
    return 58;
  case "mother":
    return 56;
  return unknown;

JS knows several values which can be false, e.g. false, 0, "", undefined, null). For example, a function returns by default undefined. To test for false use the === identity operator. The == equality operator will do type conversion to test for equality. If possible use the faster and better === strict equality operator which will test for identity (see Comparison operators (opens new window)).

Under the hood, javascript has its own ways of doing things. For example arrays:

function doIt() {
  var a = [] // empty arrays
  a.push(10) // addend number on arrays
  a.push("Monkey") // append string on arrays
  console.log(a.length) // prints 2
  a[0] // returns 10
  a[1] // returns Monkey
  a[2] // returns undefined
  a[99] = "String" // a valid assignment
  console.log(a.length) // prints 100
  a[98] // contains the value undefined

Also for people coming from C++ or Java which are used to an OO language JS just works differently. JS is not purely an OO language it is a so-called prototype based language. Each object has a prototype object. An object is created based on his prototype object. Please read more about this in the book Javascript the Good Parts by Douglas Crockford (opens new window).

To test some small JS snippets you can use the online JS Console (opens new window) or just build a little piece of QML code:

import QtQuick 2.5

Item {
  function runJS() {
    console.log("Your JS code goes here");
  Component.onCompleted: {