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JavaScript is a multi-paradigm, dynamic language with types and operators, standard built-in objects, and methods. Its syntax is based on the Java and C languages — many structures from those languages apply to JavaScript as well. JavaScript supports object-oriented programming with object prototypes, instead of classes. JavaScript also supports functional programming — because they are objects, functions may be stored in variables and passed around like any other object.

The basics

Javascript types

JavaScript's types are:

  • Number
  • String
  • Boolean
  • Symbol (new in ES2015)
  • Object
    • Function
    • Array
    • Date
    • RegExp
  • null
  • undefined


Numbers in JavaScript are double-precision 64-bit format IEEE 754 values. There's no such thing as an integer in JavaScript, so you have to be a little careful with your arithmetic.

The standard arithmetic operators are supported, including addition, subtraction, modulus (or remainder) arithmetic, and so forth. Use the Math object when in need of more advanced mathematical functions and constants.

It supports NaN for Not a Number which can be tested with isNaN() and Infinity which can be tested with isFinite().

JavaScript distinguishes between null and undefined, which indicates an uninitialized variable.

Convert a string to an integer

Use the built-in parseInt() function. It takes the base for the conversion as an optional but recommended second argument.

parseInt('123', 10); // 123
parseInt('010', 10); // 10

Convert a string into a float

Use the built-in parseFloat() function. Unlike parseInt() , parseFloat() always uses base 10.


Strings in JavaScript are sequences of Unicode characters (UTF-16) which support several methods.

Find the length of a string

'hello'.length; // 5


JavaScript has a boolean type, with possible values true and false. Any value will be converted when necessary to a boolean according to the following rules:

  • false, 0, empty strings (""), NaN, null, and undefined all become false.
  • All other values become true.

Boolean operations are also supported:

  • and: &&
  • or: ||
  • not: !


New variables in JavaScript are declared using one of three keywords: let, const, or var.

  • let is used to declare block-level variables.

    let a;
    let name = 'Simon';

    The declared variable is available from the block it is enclosed in.

    // myLetVariable is *not* visible out here
    for (let myLetVariable = 0; myLetVariable < 5; myLetVariable++) {
      // myLetVariable is only visible in here
    // myLetVariable is *not* visible out here
  • const is used to declare variables whose values are never intended to change. The variable is available from the block it is declared in.

    const Pi = 3.14; // variable Pi is set
    Pi = 1; // will throw an error because you cannot change a constant variable.
    * var is the most common declarative keyword. It does not have the restrictions that the other two keywords have. If you declare a variable without assigning any value to it, its type is undefined.

    // myVarVariable *is* visible out here
    for (var myVarVariable = 0; myVarVariable < 5; myVarVariable++) {
      // myVarVariable is visible to the whole function
    // myVarVariable *is* visible out here


Numeric operators:

  • +, both for numbers and strings.
  • -
  • *
  • /
  • %, which is the remainder operator.
  • =, to assign values.
  • +=
  • -=
  • ++
  • --

Comparison operators:

  • <
  • >
  • <=
  • >=
  • ==, performs type coercion if you give it different types, with sometimes interesting results

    123 == '123'; // true
    1 == true; // true

    To avoid type coercion, use the triple-equals operator:

    123 === '123'; // false
    1 === true;    // false
    * != and !==.

Control structures

If conditionals

var name = 'kittens';
if (name == 'puppies') {
  name += ' woof';
} else if (name == 'kittens') {
  name += ' meow';
} else {
  name += '!';
name == 'kittens meow';

You can use the conditional ternary operator instead.

It's defined by a condition followed by a question mark ?, then an expression to execute if the condition is truthy followed by a colon :, and finally the expression to execute if the condition is falsy.

condition ? exprIfTrue : exprIfFalse

function getFee(isMember) {
  return (isMember ? '$2.00' : '$10.00');

// expected output: "$2.00"

// expected output: "$10.00"

// expected output: "$10.00"

Switch cases

switch (action) {
  case 'draw':
  case 'eat':
If you don't add a break statement, execution will "fall through" to the next level. The default clause is optional

While loops

while (true) {
  // an infinite loop!

var input;
do {
  input = get_input();
} while (inputIsNotValid(input));

For loops

It has several types of for loops:

  • Classic for:

    for (var i = 0; i < 5; i++) {
      // Will execute 5 times
  • for...of.

    for (let value of array) {
      // do something with value


    for (let property in object) {
      // do something with object property


Objects can be thought of as simple collections of name-value pairs, such as Python dictionaries.

var obj2 = {};
var obj = {
  name: 'Carrot',
  for: 'Max', // 'for' is a reserved word, use '_for' instead.
  details: {
    color: 'orange',
    size: 12

Attribute access can be chained together:

obj.details.color; // orange
obj['details']['size']; // 12

The following example creates an object prototype(Person) and an instance of that prototype(you).

function Person(name, age) { = name;
  this.age = age;

// Define an object
var you = new Person('You', 24);
// We are creating a new person named "You" aged 24.

The this keyword

The this keyword refers to different objects depending on how it is used:

  • In an object method, this refers to the object.

    const person = {
      firstName: "John",
      lastName : "Doe",
      id       : 5566,
      fullName : function() {
        return this.firstName + " " + this.lastName;

  • Alone, this refers to the global object. In a browser window the global object is [object Window].

    let x = this;
  • In a function, this refers to the global object.

  • In a function, in strict mode, this is undefined.
  • In an event, this refers to the element that received the event.

    <button onclick="'none'">
      Click to Remove Me!
  • Methods like call(), apply(), and bind() can refer this to any object.


Arrays can be thought of as Python lists. They work very much like regular objects but with their own properties and methods, such as length, which returns one more than the highest index in the array.

var a = new Array();
a[0] = 'dog';
a[1] = 'cat';
a[2] = 'hen';
// or

var a = ['dog', 'cat', 'hen'];

a.length; // 3

Iterate over the values of an array

for (const currentValue of a) {
  // Do something with currentValue

// or

for (var i = 0; i < a.length; i++) {
  // Do something with a[i]

Append an item to an array

If you want to alter the original array use push() although, it's better to use concat() as it doesn't mutate the original array.


Apply a function to the elements of an array

const numbers = [1, 2, 3];
const doubled = => x * 2); // [2, 4, 6]

Filter the contents of an array

The filter() method creates a new array filled with elements that pass a test provided by a function.

The filter() method does not execute the function for empty elements.

The filter() method does not change the original array.

For example:

const ages = [32, 33, 16, 40];
const result = ages.filter(checkAdult);

function checkAdult(age) {
  return age >= 18;

Array useful methods



function add(x, y) {
  var total = x + y;
  return total;

Functions have an arguments array holding all of the values passed to the function.

To save typing and avoid the confusing behavior of this,it is recommended to use the arrow function syntax for event handlers.

So instead of

<button className="square" onClick={function() { alert('click'); }}>

It's better to use

<button className="square" onClick={() => alert('click')}>
Notice how with onClick={() => alert('click')}, the function is passed as the onClick prop.

Another example, from this code:

hello = function() {
  return "Hello World!";

You get:

hello = () => "Hello World!";

If you have parameters, you pass them inside the parentheses:

hello = (val) => "Hello " + val;

Define variable number of arguments

function avg(...args) {
  var sum = 0;
  for (let value of args) {
    sum += value;
  return sum / args.length;

avg(2, 3, 4, 5); // 3.5

Function callbacks

A callback is a function passed as an argument to another function.

Using a callback, you could call the calculator function myCalculator with a callback, and let the calculator function run the callback after the calculation is finished:

function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;

function myCalculator(num1, num2, myCallback) {
  let sum = num1 + num2;

myCalculator(5, 5, myDisplayer);

Custom objects

JavaScript is a prototype-based language that contains no class statement. Instead, JavaScript uses functions as classes.

function makePerson(first, last) {
  return {
    first: first,
    last: last,
    fullName: function() {
      return this.first + ' ' + this.last;
    fullNameReversed: function() {
      return this.last + ', ' + this.first;
var s = makePerson('Simon', 'Willison');
s.fullName(); // "Simon Willison"
s.fullNameReversed(); // "Willison, Simon"

Used inside a function, this refers to the current object. If you called it using dot notation or bracket notation on an object, that object becomes this. If dot notation wasn't used for the call, this refers to the global object.

Which makes this is a frequent cause of mistakes. For example:

var s = makePerson('Simon', 'Willison');
var fullName = s.fullName;
fullName(); // undefined undefined

When calling fullName() alone, without using s.fullName(), this is bound to the global object. Since there are no global variables called first or last we get undefined for each one.

Constructor functions

We can take advantage of the this keyword to improve the makePerson function:

function Person(first, last) {
  this.first = first;
  this.last = last;
  this.fullName = function() {
    return this.first + ' ' + this.last;
  this.fullNameReversed = function() {
    return this.last + ', ' + this.first;
var s = new Person('Simon', 'Willison');

new is strongly related to this. It creates a brand new empty object, and then calls the function specified, with this set to that new object. Notice though that the function specified with this does not return a value but merely modifies the this object. It's new that returns the this object to the calling site. Functions that are designed to be called by new are called constructor functions. Common practice is to capitalize these functions as a reminder to call them with new.

Every time we create a person object we are creating two brand new function objects within it, to avoid it, use shared functions.

function Person(first, last) {
  this.first = first;
  this.last = last;
Person.prototype.fullName = function() {
  return this.first + ' ' + this.last;
Person.prototype.fullNameReversed = function() {
  return this.last + ', ' + this.first;
Person.prototype is an object shared by all instances of Person. any time you attempt to access a property of Person that isn't set, JavaScript will check Person.prototype to see if that property exists there instead. As a result, anything assigned to Person.prototype becomes available to all instances of that constructor via the this object. So it's easy to add extra methods to existing objects at runtime:

var s = new Person('Simon', 'Willison');
s.firstNameCaps(); // TypeError on line 1: s.firstNameCaps is not a function

Person.prototype.firstNameCaps = function() {
  return this.first.toUpperCase();
s.firstNameCaps(); // "SIMON"

Split code for readability

To split a line into several, parentheses may be used to avoid the insertion of semicolons.

  renderSquare(i) {
    return (
        onClick={() => this.handleClick(i)}

Coalescent operator

Is similar to the Logical OR operator (||), except instead of relying on truthy/falsy values, it relies on "nullish" values (there are only 2 nullish values, null and undefined).

This means it's safer to use when you treat falsy values like 0 as valid.

Similar to Logical OR, it functions as a control-flow operator; it evaluates to the first not-nullish value.

It was introduced in Chrome 80 / Firefox 72 / Safari 13.1. It has no IE support.

console.log(4 ?? 5);
// 4, since neither value is nullish
console.log(null ?? 10);
// 10, since 'null' is nullish
console.log(undefined ?? 0);
// 0, since 'undefined' is nullish
// Here's a case where it differs from
// Logical OR (||):
console.log(0 ?? 5); // 0
console.log(0 || 5); // 5

Interacting with HTML

You can find HTML elements with the next document properties:

Property Description
document.anchors Returns all <a> elements that have a name attribute
document.baseURI Returns the absolute base URI of the document
document.body Returns the <body> element
document.cookie Returns the document's cookie
document.doctype Returns the document's doctype
document.documentElement Returns the <html> element
document.documentMode Returns the mode used by the browser
document.documentURI Returns the URI of the document
document.domain Returns the domain name of the document server
document.embeds Returns all <embed> elements
document.forms Returns all <form> elements
document.head Returns the <head> element
document.images Returns all <img> elements
document.implementation Returns the DOM implementation
document.inputEncoding Returns the document's encoding (character set)
document.lastModified Returns the date and time the document was updated
document.links Returns all <area> and <a> elements that have a href attribute
document.readyState Returns the (loading) status of the document
document.referrer Returns the URI of the referrer (the linking document)
document.scripts Returns all <script> elements
document.strictErrorChecking Returns if error checking is enforced
document.title Returns the <title> element
document.URL Returns the complete URL of the document

How to add JavaScript to HTML

In HTML, JavaScript code is inserted between <script> and </script> tags.

document.getElementById("demo").innerHTML = "My First JavaScript";

That will be run as the page is loaded.

Scripts can be placed in the <body>, or in the <head> section of an HTML page, or in both.


"Placing scripts at the bottom of the `<body>` element improves the display speed, because script interpretation slows down the display."

A JavaScript function is a block of JavaScript code, that can be executed when "called" for.

For example, a function can be called when an event occurs, like when the user clicks a button.

External JavaScript

Scripts can also be placed in external files:

File: myScript.js

function myFunction() {
  document.getElementById("demo").innerHTML = "Paragraph changed.";

External scripts are practical when the same code is used in many different web pages.

To use an external script, put the name of the script file in the src (source) attribute of a <script> tag:

<script src="myScript.js"></script>

Placing scripts in external files has some advantages:

  • It separates HTML and code.
  • It makes HTML and JavaScript easier to read and maintain.
  • Cached JavaScript files can speed up page loads.

HTML content

One of many JavaScript HTML methods is getElementById().

The example below "finds" an HTML element (with id="demo"), and changes the element content (innerHTML) to "Hello JavaScript":

document.getElementById("demo").innerHTML = "Hello JavaScript";

It will transform:

<p id="demo">JavaScript can change HTML content.</p>


<p id="demo">Hello JavaScript</p>

You can also use getElementsByTagName or getElementsByClassName. Or if you want to find all HTML elements that match a specified CSS selector (id, class names, types, attributes, values of attributes, etc), use the querySelectorAll() method.

This example returns a list of all <p> elements with class="intro".

const x = document.querySelectorAll("p.intro");

HTML attributes

JavaScript can also change HTML attribute values. In this example JavaScript changes the value of the src (source) attribute of an <img> tag:

<button onclick="document.getElementById('myImage').src='pic_bulbon.gif'">Turn on the light</button>

<img id="myImage" src="pic_bulboff.gif" style="width:100px">

<button onclick="document.getElementById('myImage').src='pic_bulboff.gif'">Turn off the light</button>

Other attribute methods are:

Method Description
document.createElement(element) Create an HTML element
document.removeChild(element) Remove an HTML element
document.appendChild(element) Add an HTML element
document.replaceChild(new, old) Replace an HTML element


Changing the style of an HTML element, is a variant of changing an HTML attribute:

document.getElementById("demo").style.fontSize = "35px";

Hiding or showing HTML elements

Hiding HTML elements can be done by changing the display style:

document.getElementById("demo").style.display = "none";

Showing hidden HTML elements can also be done by changing the display style:

document.getElementById("demo").style.display = "block";

Displaying data

JavaScript can "display" data in different ways:

  • Writing into an HTML element, using innerHTML.
  • Writing into the HTML output using document.write().

    <h1>My First Web Page</h1>
    <p>My first paragraph.</p>
    document.write(5 + 6);

    Using document.write() after an HTML document is loaded, will delete all existing HTML.

  • Writing into an alert box, using window.alert().

  • Writing into the browser console, using console.log(). Useful for debugging.


An HTML event can be something the browser does, or something a user does.

Here are some examples of HTML events:

  • An HTML web page has finished loading
  • An HTML input field was changed
  • An HTML button was clicked

Often, when events happen, you may want to do something.

JavaScript lets you execute code when events are detected.

HTML allows event handler attributes, with JavaScript code, to be added to HTML elements.

<element event='some JavaScript'>

In the following example, an onclick attribute (with code), is added to a <button> element:

<button onclick="document.getElementById('demo').innerHTML = Date()">The time is?</button>

In the example above, the JavaScript code changes the content of the element with id="demo".

In the next example, the code changes the content of its own element (using this.innerHTML):

<button onclick="this.innerHTML = Date()">The time is?</button>

JavaScript code is often several lines long. It is more common to see event attributes calling functions:

<button onclick="displayDate()">The time is?</button>

Common HTML Events

Here is a list of some common HTML events:

Event Description
onchange An HTML element has been changed
onclick The user clicks an HTML element
onmouseover The user moves the mouse over an HTML element
onmouseout The user moves the mouse away from an HTML element
onmousedown The user is pressing the click button
onmouseup The user is releasing the click button
onkeydown The user pushes a keyboard key
onload The browser has finished loading the page

Event handlers

Event handlers can be used to handle and verify user input, user actions, and browser actions:

  • Things that should be done every time a page loads
  • Things that should be done when the page is closed
  • Action that should be performed when a user clicks a button
  • Content that should be verified when a user inputs data

Many different methods can be used to let JavaScript work with events:

  • HTML event attributes can execute JavaScript code directly
  • HTML event attributes can call JavaScript functions
  • You can assign your own event handler functions to HTML elements
  • You can prevent events from being sent or being handled

JSON support

JSON is a format for storing and transporting data.

A common use of JSON is to read data from a web server, and display the data in a web page.

For simplicity, this can be demonstrated using a string as input.

First, create a JavaScript string containing JSON syntax:

let text = '{ "employees" : [' +
'{ "firstName":"John" , "lastName":"Doe" },' +
'{ "firstName":"Anna" , "lastName":"Smith" },' +
'{ "firstName":"Peter" , "lastName":"Jones" } ]}';

Then, use the JavaScript built-in function JSON.parse() to convert the string into a JavaScript object:

const obj = JSON.parse(text);

Finally, use the new JavaScript object in your page:

<p id="demo"></p>

document.getElementById("demo").innerHTML =
obj.employees[1].firstName + " " + obj.employees[1].lastName;

Async JavaScript

Timing events

The window object allows execution of code at specified time intervals.

These time intervals are called timing events.

The two key methods to use with JavaScript are:

  • setTimeout(function, milliseconds): Executes a function, after waiting a specified number of milliseconds.
  • setInterval(function, milliseconds): Same as setTimeout(), but repeats the execution of the function continuously.

For example to click a button. Wait 3 seconds, and the page will alert "Hello":

 <button onclick="setTimeout(myFunction, 3000)">Try it</button>

function myFunction() {

The window.clearTimeout(timeoutVariable) method stops the execution of the function specified in setTimeout().

myVar = setTimeout(function, milliseconds);

To stop a setInterval use the clearInterval method.


A JavaScript Promise object contains both the producing code and calls to the consuming code, where:

  • "Producing code" is code that can take some time
  • "Consuming code" is code that must wait for the result

The syntax of a Promise is kind of difficult to understand, but bear with me:

let myPromise = new Promise(function(myResolve, myReject) {
// "Producing Code" (May take some time)

  myResolve(); // when successful
  myReject();  // when error

// "Consuming Code" (Must wait for a fulfilled Promise)
  function(value) { /* code if successful */ },
  function(error) { /* code if some error */ }

When the producing code obtains the result, it should call one of the two callbacks:

  • Success then calls myResolve(result value)
  • Error then calls myReject(error object)

For example:

function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;

let myPromise = new Promise(function(myResolve, myReject) {
  let x = 0;

// The producing code (this may take some time)

  if (x == 0) {
  } else {

  function(value) {myDisplayer(value);},
  function(error) {myDisplayer(error);}

Promise object properties

A JavaScript Promise object can be:

  • Pending
  • Fulfilled
  • Rejected

The Promise object supports two properties: state and result, which can't be accessed directly.

  • While a Promise object is pending (working), the result is undefined.
  • When a Promise object is fulfilled, the result is a value.
  • When a Promise object is rejected, the result is an error object.

Waiting for a timeout example

Example Using Callback:

setTimeout(function() { myFunction("I love You !!!"); }, 3000);

function myFunction(value) {
  document.getElementById("demo").innerHTML = value;

Example Using Promise

let myPromise = new Promise(function(myResolve, myReject) {
  setTimeout(function() { myResolve("I love You !!"); }, 3000);

myPromise.then(function(value) {
  document.getElementById("demo").innerHTML = value;


async and await make promises easier to write.

async makes a function return a Promise, while await makes a function wait for a Promise.

Async syntax

For example

async function myFunction() {
  return "Hello";

Is the same as:

function myFunction() {
  return Promise.resolve("Hello");

Here is how to use the Promise:

  function(value) {myDisplayer(value);},
  function(error) {myDisplayer(error);}

If you only expect a normal value, skip the function(error)... line.

Await syntax

The keyword await before a function makes the function wait for a promise:

let value = await promise;

The await keyword can only be used inside an async function.

For example the next code will update the content of demo with I love you !! instantly:

async function myDisplay() {
  let myPromise = new Promise(function(resolve, reject) {
    resolve("I love You !!");
  document.getElementById("demo").innerHTML = await myPromise;


But it could have a timeout

async function myDisplay() {
  let myPromise = new Promise(function(resolve) {
    setTimeout(function() {resolve("I love You !!");}, 3000);
  document.getElementById("demo").innerHTML = await myPromise;



Cookies are data, stored in small text files, on your computer. They hold a very small amount of data at a maximum capacity of 4KB.

When a web server has sent a web page to a browser, the connection is shut down, and the server forgets everything about the user.

Cookies were invented to solve the problem "how to remember information about the user":

  • When a user visits a web page, his/her name can be stored in a cookie.
  • Next time the user visits the page, the cookie "remembers" his/her name.

There are two types of cookies: persistent cookies and session cookies. Session cookies do not contain an expiration date. Instead, they are stored only as long as the browser or tab is open. As soon as the browser is closed, they are permanently lost. Persistent cookies do have an expiration date. These cookies are stored on the user’s disk until the expiration date and then permanently deleted.


"Think if for your case ithe's better to use [Web storage](#web-storage) instead"

Cookies are saved in name-value pairs like:

username = John Doe

When a browser requests a web page from a server, cookies belonging to the page are added to the request. This way the server gets the necessary data to "remember" information about users.

JavaScript can create, read, and delete cookies with the document.cookie property.

With JavaScript, a cookie can be created like this:

document.cookie = "username=John Doe";

You can also add an expiry date (in UTC time). By default, the cookie is deleted when the browser is closed:

document.cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC";

With a path parameter, you can tell the browser what path the cookie belongs to. By default, the cookie belongs to the current page.

document.cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/";

With JavaScript, cookies can be read like this:

let x = document.cookie;

document.cookie will return all cookies in one string much like: cookie1=value; cookie2=value; cookie3=value;. It looks like a normal text string. But it is not.

If you want to find the value of one specified cookie, you must write a JavaScript function that searches for the cookie value in the cookie string.

With JavaScript, you can change a cookie the same way as you create it:

document.cookie = "username=John Smith; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/";

The old cookie is overwritten.

Just set the expires parameter to a past date:

document.cookie = "username=; expires=Thu, 01 Jan 1970 00:00:00 UTC; path=/;";

In the example to follow, we will create a cookie that stores the name of a visitor.

The first time a visitor arrives to the web page, he/she will be asked to fill in his/her name. The name is then stored in a cookie.

The next time the visitor arrives at the same page, he/she will get a welcome message.

function setCookie(cname, cvalue, exdays) {
  const d = new Date();
  d.setTime(d.getTime() + (exdays*24*60*60*1000));
  let expires = "expires="+ d.toUTCString();
  document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";
function getCookie(cname) {
  let name = cname + "=";
  let decodedCookie = decodeURIComponent(document.cookie);
  let ca = decodedCookie.split(';');
  for(let i = 0; i <ca.length; i++) {
    let c = ca[i];
    while (c.charAt(0) == ' ') {
      c = c.substring(1);
    if (c.indexOf(name) == 0) {
      return c.substring(name.length, c.length);
  return "";


  • The function take the cookiename as parameter cname.
  • Creates a variable name with the text to search for cname + "=".
  • Decodes the cookie string, to handle cookies with special characters like $.
  • Split document.cookie on semicolons into an array called ca.
  • Loop through the ca array and read out each value c.
  • If the cookie is found c.indexOf(name) == 0, return the value of the cookie c.substring(name.length, c.length.
  • If the cookie is not found, return "".

If the cookie is set it will display a greeting.

If the cookie is not set, it will display a prompt box, asking for the name of the user, and stores the username cookie for 365 days, by calling the setCookie function:

function checkCookie() {
  let username = getCookie("username");
  if (username != "") {
   alert("Welcome again " + username);
  } else {
    username = prompt("Please enter your name:", "");
    if (username != "" && username != null) {
      setCookie("username", username, 365);

The html code of the page would be:

<body onload="checkCookie()"></body>

Web Storage

The Web Storage API is a simple syntax for storing and retrieving data in the browser. There are two storage objects localStorage and sessionStorage.

localStorage.setItem("name", "John Doe");

The storage object properties and methods are:

Property/Method Description
key(n) Returns the name of the nth key in the storage
length Returns the number of data items stored in the Storage object
getItem(keyname) Returns the value of the specified key name
setItem(keyname, value) Adds or updates the key to the storage
removeItem(keyname) Removes that key from the storage
clear() Empty all key out of the storage

sessionStorage is identical to the localStorage but it only stores the data for one session, so the data will be deleted when the browser is closed.

Introduced by HTML5, it has replaced many of the cookies uses. This is because LocalStorage has a lot of advantages over cookies. One of the most important differences is that unlike with cookies, data does not have to be sent back and forth with every HTTP request. This reduces the overall traffic between the client and the server and the amount of wasted bandwidth. This is because data is stored on the user’s local disk and is not destroyed or cleared by the loss of an internet connection. Also, LocalStorage can hold up to 5MB of information which is a whole lot more than the 4KB that cookies hold.

LocalStorage behaves more like persistent cookies in terms of expiration. Data is not automatically destroyed unless it is cleared through Javascript code. This can be good for larger bits of data that need to be stored for longer periods of time. Also, with LocalStorage you can not only store strings but also Javascript primitives and objects.

An example of a good use of LocalStorage might be in an application used in regions without a persistent internet connection. In order for this to be a good use of LocalStorage, the threat level of the data stored in this situation would have to be very low. To protect client privacy, it would be good to upload the data when connection is re-established and then delete the locally stored version.

In conclusion, Cookies are smaller and send server information back with every HTTP request, while LocalStorage is larger and can hold information on the client side.

Web workers

When executing scripts in an HTML page, the page becomes unresponsive until the script is finished.

A web worker is a JavaScript that runs in the background, independently of other scripts, without affecting the performance of the page. You can continue to do whatever you want: clicking, selecting things, etc., while the web worker runs in the background.

Since web workers are in external files, they do not have access to the following JavaScript objects:

  • The window object
  • The document object
  • The parent object

Before creating a web worker check whether the user's browser supports it:

if (typeof(Worker) !== "undefined") {
  // Yes! Web worker support!
  // Some code.....
} else {
  // Sorry! No Web Worker support..

Now, let's create our web worker in an external JavaScript.

Here, we create a script that counts. The script is stored in the demo_workers.js file:

let i = 0;

function timedCount() {
  i ++;


The important part of the code above is the postMessage() method, which is used to post a message back to the HTML page.

Create a Web Worker object

Now that we have the web worker file, we need to call it from an HTML page.

The following lines checks if the worker already exists, if not it creates a new web worker object and runs the code in demo_workers.js:

if (typeof(w) == "undefined") {
  w = new Worker("demo_workers.js");

Then we can send and receive messages from the web worker.

Add an onmessage event listener to the web worker.

w.onmessage = function(event){
  document.getElementById("result").innerHTML =;

When the web worker posts a message, the code within the event listener is executed. The data from the web worker is stored in

Terminate a Web Worker

When a web worker object is created, it will continue to listen for messages (even after the external script is finished) until it is terminated.

To terminate a web worker, and free browser/computer resources, use the terminate() method:


Reuse the Web Worker

If you set the worker variable to undefined, after it has been terminated, you can reuse the code:

w = undefined;

Full web worker example code

 <!DOCTYPE html>

<p>Count numbers: <output id="result"></output></p>
<button onclick="startWorker()">Start Worker</button>
<button onclick="stopWorker()">Stop Worker</button>

let w;

function startWorker() {
  if (typeof(w) == "undefined") {
    w = new Worker("demo_workers.js");
  w.onmessage = function(event) {
    document.getElementById("result").innerHTML =;

function stopWorker() {
  w = undefined;


Interacting with external APIs

The Fetch API interface allows web browser to make HTTP requests to web servers.

The example below fetches a file and displays the content:

<p id="demo">Fetch a file to change this text.</p>

let file = "fetch_info.txt"

fetch (file)
.then(x => x.text())
.then(y => document.getElementById("demo").innerHTML = y);


Where the content of fetch_info.txt is:

<h1>Fetch API</h1>
<p>The Fetch API interface allows web browser to make HTTP requests to web servers.</p>

Since Fetch is based on async and await, the example above might be easier to understand like this:

async function getText(file) {
  let x = await fetch(file);
  let y = await x.text();

Or even better: Use understandable names instead of x and y:

async function getText(file) {
  let myObject = await fetch(file);
  let myText = await myObject.text();

The first .then() resolves the promise into a response object. To be able to view the content of this object the response is then converted using a .json() method. This json() also returns a promise so another .then() is necessary.

Doing a POST

We can first include the settings such as header and request method in a object.

var jsonObj = {};
jsonObj.firstParam = "first";
jsonObj.secondParam = 2;
jsonObj.thirdParam = true;

var options = {
    method: 'POST',
    header: new Headers({
        "Content-Type": "application/json",
    body: JSON.stringify(jsonObj)

var url = http://localhost:8080/postRequest;

fetch(url, options)
.then((response) => {
    console.log("Status Code",response.status);
    //return response type such as json, blob, text, formData and arrayBuffer
    return response.json()
.then((result) => {
    //here will return whatever information from the response.
    console.log("response message from backend", result);
.catch((error) => {

Error handling

  • The try statement defines a code block to run (to try).
  • The catch statement defines a code block to handle any error.
  • The finally statement defines a code block to run regardless of the result.
  • The throw statement defines a custom error.

    throw "Too big";    // throw a text
    throw 500;          // throw a number

In this example we misspelled alert as adddlert to deliberately produce an error:

<p id="demo"></p>

try {
  adddlert("Welcome guest!");
catch(err) {
  document.getElementById("demo").innerHTML = err.message;

When an error occurs, JavaScript will normally stop and generate an error message.

JavaScript will create an Error object with two properties: name and message.

Six different values can be returned by the error name property:

Error Name Description
EvalError An error has occurred in the eval() function
RangeError A number "out of range" has occurred
ReferenceError An illegal reference has occurred
SyntaxError A syntax error has occurred
TypeError A type error has occurred
URIError An error in encodeURI() has occurred

Input validation example

This example examines input. If the value is wrong, an exception (err) is thrown.

The exception is caught by the catch statement and a custom error message is displayed:

<p>Please input a number between 5 and 10:</p>

<input id="demo" type="text">
<button type="button" onclick="myFunction()">Test Input</button>
<p id="p01"></p>

function myFunction() {
  const message = document.getElementById("p01");
  message.innerHTML = "";
  let x = document.getElementById("demo").value;
  try {
    if(x == "") throw "empty";
    if(isNaN(x)) throw "not a number";
    x = Number(x);
    if(x < 5) throw "too low";
    if(x > 10) throw "too high";
  catch(err) {
    message.innerHTML = "Input is " + err;


You can use the console.log to display JavaScript values in the debugger window:

a = 5;
b = 6;
c = a + b;

Or you can use breakpoints

let x = 15 * 5;
document.getElementById("demo").innerHTML = x;

Style guide

  • Always put spaces around operators ( = + - * / ), and after commas.
  • Always use 2 spaces for indentation of code blocks.
  • Avoid lines longer than 80 characters.
  • Always end a simple statement with a semicolon.

    const cars = ["Volvo", "Saab", "Fiat"];
    const person = {
      firstName: "John",
      lastName: "Doe",
      age: 50,
      eyeColor: "blue"
  • General rules for complex (compound) statements:

    • Put the opening bracket at the end of the first line.
    • Use one space before the opening bracket.
    • Put the closing bracket on a new line, without leading spaces.
    • Do not end a complex statement with a semicolon.
    function toCelsius(fahrenheit) {
      return (5 / 9) * (fahrenheit - 32);
    for (let i = 0; i < 5; i++) {
      x += i;
    if (time < 20) {
      greeting = "Good day";
    } else {
      greeting = "Good evening";
  • General rules for object definitions:

    • Place the opening bracket on the same line as the object name.
    • Use colon plus one space between each property and its value.
    • Use quotes around string values, not around numeric values.
    • Do not add a comma after the last property-value pair.
    • Place the closing bracket on a new line, without leading spaces.
    • Always end an object definition with a semicolon.
    const person = {
      firstName: "John",
      lastName: "Doe",
      age: 50,
      eyeColor: "blue"

Best practices

  • Minimize the use of global variables.
  • All variables used in a function should be declared as local variables.
  • It is a good coding practice to put all declarations at the top of each script or function.

    This will:

    • Give cleaner code
    • Provide a single place to look for local variables
    • Make it easier to avoid unwanted (implied) global variables
    • Reduce the possibility of unwanted re-declarations
  • Initialize variables when you declare them.

    This will:

    • Give cleaner code
    • Provide a single place to initialize variables
    • Avoid undefined values
  • Declaring objects and arrays with const will prevent any accidental change of type.

  • Don't use the new Object()

    • Use "" instead of new String().
    • Use 0 instead of new Number().
    • Use false instead of new Boolean().
    • Use {} instead of new Object().
    • Use [] instead of new Array().
    • Use /()/ instead of new RegExp().
    • Use function (){} instead of new Function().
    • Use === for comparison. The == comparison operator always converts (to matching types) before comparison.
    • Use parameter defaults. If a function is called with a missing argument, the value of the missing argument is set to undefined.

    Undefined values can break your code. It is a good habit to assign default values to arguments.

    function myFunction(x, y) {
      if (y === undefined) {
        y = 0;
    * Avoid using eval(). It's used to run text as code which represents a security problem.

  • Reduce DOM access. Accessing the HTML DOM is very slow, compared to other JavaScript statements.

    If you expect to access a DOM element several times, access it once, and use it as a local variable:

    const obj = document.getElementById("demo");
    obj.innerHTML = "Hello";
    * Keep the number of elements in the HTML DOM small. This will always improve page loading, and speed up rendering (page display), especially on smaller devices.

    Every attempt to search the DOM (like getElementsByTagName) will benefit from a smaller DOM.

  • Delay JavaScript loading. Putting your scripts at the bottom of the page body lets the browser load the page first.

    While a script is downloading, the browser will not start any other downloads. In addition all parsing and rendering activity might be blocked.

  • Avoid using the with keyword. It has a negative effect on speed. It also clutters up JavaScript scopes.