JavaScript Concept

There are two features of null you should understand : null is an empty or non-existent value and must be assigned.


let num= null;

console.log(num); // null

Undefined most typically means a variable has been declared, but not defined.


let num;

console.log(num); // undefined

In JavaScript there are only six falsy values. Both null and undefined are two of the six falsy values. Here is a full list:

  1. 0 (zero)
  2. “” (empty string)
  3. null
  4. undefined
  5. NaN (Not A Number)

Any other value in JavaScript is considered truthy.

Also in JavaScript, there are six primitive values. Both null and undefined are primitive values. Here is a full list:

  1. Null
  2. Undefined
  3. Number
  4. String
  5. Symbol

Both double equals == and triple equals === operator is used for comparing between two values on which the operator is used on. The difference between the two operators is that the double equals == will compare the values loosely, meaning that it will try to convert values with different types before comparing them. The triple equals === won’t convert values of different types. It will simply return false when comparing values of different types. To understand their differences, let’s try comparing two different values between the number value 0 and boolean value false:

console.log(0 == false); // true

console.log(0 == false); // false

The following example shows the double equal == and triple equal === conversion rules in action:

console.log(1 == true); // 1 == 1 = true

console.log(1 === true); // false

console.log(5 == “5”); // 5 == 5 = true

console.log(5 === “5”); // false

console.log(2 == true); // 2 == 1 = false

console.log(2 === true); // false

Scope determines the visibility or accessibility of a variable or other resource in the area of your code.

There’s only one Global scope in the JavaScript document. The area outside all the functions is consider the global scope and the variables defined inside the global scope can be accessed and altered in any other scopes.


var num = 5; // global scope


function getNum() {

console.log(num); // num is accessible here


getNum(); // 5

Variables declared inside the functions become Local to the function and are considered in the corresponding local scope. Every Functions has its own scope. Same variable can be used in different functions because they are bound to the respective functions and are not mutual visible.


//global scope

function f1() {

//local scope 1

function f2() {

//local scope 2



//global scope

function f3() {

//local scope


//global scope

Local scope can be divided into function scope and block scope. The concept of block scope is introduced in ECMA script 6 (ES6) together with the new ways to declare variables — const and let.

Whenever you declare a variable in a function, the variable is visible only within the function. You can’t access it outside the function. var is the keyword to define variable for a function-scope accessibility.


function f() {

var num = 10;

console.log(‘Inside function: ’, num);


f(); // inside function: 10

console.log(num); // error: num is not defined

A block scope is the area within if, switch conditions or for and while loops. Generally speaking, whenever you see {curly brackets}, it is a block. In ES6, const and let keywords allow developers to declare variables in the block scope, which means those variables exist only within the corresponding block.


function f() {

if(true) {

var num1 = 10; // exist in function scope

const num2 = 20; // exist in block scope

let num3 = 30; // exist in block scope







//result: 10, error: num2 is not defined, error: num3 is not defined.

Another point to mention is the lexical scope. Lexical scope means the children scope have the access to the variables defined in the parent scope. The children functions are lexically bound to the execution context of their parents.


function f1() {

var num1 = 10; // exist in function scope

const num2 = 20; // exist in block scope

let num3 = 30; // exist in block scope

function f2() {








//result: 10, 20, 30.

Call( ): The call() method invokes a function with a given ‘this’ value and arguments provided one by one. This means that we can call any function, and explicitly specify what ‘this’ should reference within the calling function.

Apply( ): Invokes the function and allows you to pass in arguments as an array.

Bind(): returns a new function, allowing you to pass in an array and any number of arguments.

When we use the bind() method:

1.The JS engine is creating a new invite instance and binding friend1 and friend2 as its ‘this’ variable. So basically it copies the invite function.

2.After creating a copy of the invite function it is able to call inviteFriend1( ) and inviteFriend2( ), although it wasn’t on the friend1 and friend2 object initially. It will now recognizes its properties and its methods.

Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. Whereas Bind creates a new function that will have this set to the first parameter passed to bind().

Global object

A global object is an object that always exists in the global scope.

In JavaScript, there’s always a global object defined. In a web browser, when scripts create global variables defined with the var keyword, they're created as members of the global object. (In Node.js this is not the case.) The global object's interface depends on the execution context in which the script is running. For example:

  • Code running in a Worker has a WorkerGlobalScope object as its global object.
  • Scripts running under Node.js have an object called global as their global object.

Window object in the Browser

The window object is the Global Object in the Browser. Any Global Variables or Functions can be accessed as properties of the window object.

Access Global Variables

var foo = "foobar";
foo ===; // Returns: true

After defining a Global Variable foo, we can access its value directly from the window object, by using the variable name foo as a property name of the Global Object


The global variable foo was stored in the window object, like this:

foo: "foobar"

Access Global Functiuons

function greeting() {
window.greeting(); // It is the same as the normal invoking: greeting();

The example above explains how Global Functions are stored as properties in the window object. We created a Global Function called greeting, then invoked it using the window object.


The global function greeting was stored in the window object, like this:

greeting: function greeting() {


For a long time, the web platform has offered JavaScript programmers a number of functions that allow them to asynchronously execute code after a certain time interval has elapsed, and to repeatedly execute a block of code asynchronously until you tell it to stop.

These functions are:

setTimeout()Execute a specified block of code once after a specified time has elapsed.setInterval()Execute a specified block of code repeatedly with a fixed time delay between each call.requestAnimationFrame()The modern version of setInterval(). Executes a specified block of code before the browser next repaints the display, allowing an animation to be run at a suitable framerate regardless of the environment it is being run in.

The asynchronous code set up by these functions runs on the main thread (after their specified timer has elapsed).

It’s important to know that you can (and often will) run other code before a setTimeout() call executes, or between iterations of setInterval(). Depending on how processor-intensive these operations are, they can delay your async code even further, as any async code will execute only after the main thread is available. (In other words, when the stack is empty.) You will learn more on this matter as you progress through this article.

In any case, these functions are used for running constant animations and other background processing on a web site or application. In the following sections we will show you how they can be used.


As we said before, setTimeout() executes a particular block of code once after a specified time has elapsed. It takes the following parameters:

  • A number representing the time interval in milliseconds (1000 milliseconds equals 1 second) to wait before executing the code. If you specify a value of 0 (or omit the value), the function will run as soon as possible. (See the note below on why it runs "as soon as possible" and not "immediately".) More on why you might want to do this later.
  • Zero or more values that represent any parameters you want to pass to the function when it is run.

NOTE: The specified amount of time (or the delay) is not the guaranteed time to execution, but rather the minimum time to execution. The callbacks you pass to these functions cannot run until the stack on the main thread is empty.

As a consequence, code like setTimeout(fn, 0) will execute as soon as the stack is empty, not immediately. If you execute code like setTimeout(fn, 0) but then immediately after run a loop that counts from 1 to 10 billion, your callback will be executed after a few seconds.

In the following example, the browser will wait two seconds before executing the anonymous function, then will display the alert message (see it running live, and see the source code):

let myGreeting = setTimeout(() => {
alert('Hello, Mr. Universe!');
}, 2000);

The functions you specify don’t have to be anonymous. You can give your function a name, and even define it somewhere else and pass a function reference to the setTimeout(). The following two versions of the code snippet are equivalent to the first one:

// With a named function
let myGreeting = setTimeout(function sayHi() {
alert('Hello, Mr. Universe!');
}, 2000);
// With a function defined separately
function sayHi() {
alert('Hello Mr. Universe!');
let myGreeting = setTimeout(sayHi, 2000);

That can be useful if you have a function that needs to be called both from a timeout and in response to an event, for example. But it can also just help keep your code tidy, especially if the timeout callback is more than a few lines of code.

setTimeout() returns an identifier value that can be used to refer to the timeout later, such as when you want to stop it. See Clearing timeouts (below) to learn how to do that.

Passing parameters to a setTimeout() function

Any parameters that you want to pass to the function being run inside the setTimeout() must be passed to it as additional parameters at the end of the list.

For example, you could refactor the previous function so that it will say hi to whatever person’s name is passed to it:

function sayHi(who) {
alert(`Hello ${who}!`);

Now, you can pass the name of the person into the setTimeout() call as a third parameter:

let myGreeting = setTimeout(sayHi, 2000, 'Mr. Universe');

Cleariung timeouts

Finally, if a timeout has been created, you can cancel it before the specified time has elapsed by calling clearTimeout(), passing it the identifier of the setTimeout() call as a parameter. So to cancel our above timeout, you'd do this:


Note: See greeter-app.html for a slightly more involved demo that allows you to set the name of the person to say hello to in a form, and cancel the greeting using a separate button (see the source code also).


setTimeout() works perfectly when you need to run code once after a set period of time. But what happens when you need to run the code over and over again—for example, in the case of an animation?

This is where setInterval() comes in. This works in a very similar way to setTimeout(), except that the function you pass as the first parameter is executed repeatedly at no less than the number of milliseconds given by the second parameter apart, rather than once. You can also pass any parameters required by the function being executed as subsequent parameters of the setInterval() call.

Let’s look at an example. The following function creates a new Date() object, extracts a time string out of it using toLocaleTimeString(), and then displays it in the UI. It then runs the function once per second using setInterval(), creating the effect of a digital clock that updates once per second (see this live, and also see the source):

function displayTime() {
let date = new Date();
let time = date.toLocaleTimeString();
document.getElementById('demo').textContent = time;
const createClock = setInterval(displayTime, 1000);

Just like setTimeout(), setInterval() returns an identifying value you can use later when you need to clear the interval.

Clearing intervals

setInterval() keeps running a task forever, unless you do something about it. You'll probably want a way to stop such tasks, otherwise you may end up getting errors when the browser can't complete any further versions of the task, or if the animation being handled by the task has finished. You can do this the same way you stop timeouts — by passing the identifier returned by the setInterval() call to the clearInterval() function:

const myInterval = setInterval(myFunction, 2000);clearInterval(myInterval);

Active learning: Creating your own stopwatch!

With this all said, we’ve got a challenge for you. Take a copy of our setInterval-clock.html example, and modify it to create your own simple stopwatch.

You need to display a time as before, but in this example, you need:

  • A “Stop” button to pause/stop it.
  • A “Reset” button to reset the time back to 0.
  • The time display to show the number of seconds elapsed, rather than the actual time.

Here’s a few hints for you:

  • You probably want to create a variable that starts at 0, then increments by one every second using a constant loop.
  • It is easier to create this example without using a Date() object, like we've done in our version, but less accurate — you can't guarantee that the callback will fire after exactly 1000ms. A more accurate way would be to run startTime = to get a timestamp of exactly when the user clicked the start button, and then do - startTime to get the number of milliseconds after the start button was clicked.
  • You also want to calculate the number of hours, minutes, and seconds as separate values, and then show them together in a string after each loop iteration. From the second counter, you can work out each of these.
  • How would you calculate them? Have a think about it:
  • The number of seconds in an hour is 3600.
  • The number of minutes will be the amount of seconds left over when all of the hours have been removed, divided by 60.
  • The number of seconds will be the amount of seconds left over when all of the minutes have been removed.
  • You’ll want to include a leading zero on your display values if the amount is less than 10, so it looks more like a traditional clock/watch.
  • To pause the stopwatch, you’ll want to clear the interval. To reset it, you’ll want to set the counter back to 0, clear the interval, and then immediately update the display.
  • You probably ought to disable the start button after pressing it once, and enable it again after you’ve stopped/reset it. Otherwise multiple presses of the start button will apply multiple setInterval()s to the clock, leading to wrong behavior.

Note: If you get stuck, you can find our version here (see the source code also).

Things to keep in mond about setTimeout() and setInterval()

There are a few things to keep in mind when working with setTimeout() and setInterval(). Let's review these now.

Recursive timeouts

There is another way to use setTimeout(): you can call it recursively to run the same code repeatedly, instead of using setInterval().

The below example uses a recursive setTimeout() to run the passed function every 100 milliseconds:

let i = 1;setTimeout(function run() {
setTimeout(run, 100);
}, 100);

Compare the above example to the following one — this uses setInterval() to accomplish the same effect:

let i = 1;setInterval(function run() {
}, 100);

How do recursive setTimeout() and setInterval() differ?

The difference between the two versions of the above code is a subtle one.

  • The example using setInterval() does things somewhat differently. The interval you chose includes the time taken to execute the code you want to run in. Let's say that the code takes 40 milliseconds to run — the interval then ends up being only 60 milliseconds.
  • When using setTimeout() recursively, each iteration can calculate a different delay before running the next iteration. In other words, the value of the second parameter can specify a different time in milliseconds to wait before running the code again.

When your code has the potential to take longer to run than the time interval you’ve assigned, it’s better to use recursive setTimeout() — this will keep the time interval constant between executions regardless of how long the code takes to execute, and you won't get errors.

Immidiate timeouts

Using 0 as the value for setTimeout() schedules the execution of the specified callback function as soon as possible but only after the main code thread has been run.

For instance, the code below (see it live) outputs an alert containing "Hello", then an alert containing "World" as soon as you click OK on the first alert.

setTimeout(function() {
}, 0);

This can be useful in cases where you want to set a block of code to run as soon as all of the main thread has finished running — put it on the async event loop, so it will run straight afterwards.

Clearing with clearTimeout() ohr clearInterval()

clearTimeout() and clearInterval() both use the same list of entries to clear from. Interestingly enough, this means that you can use either method to clear a setTimeout() or setInterval().

For consistency, you should use clearTimeout() to clear setTimeout() entries and clearInterval() to clear setInterval() entries. This will help to avoid confusion.


requestAnimationFrame() is a specialized enqueueing function created for running animations efficiently in the browser. It runs a specified block of code before the browser next repaints the display, allowing the execution to be paired with the device's display frame rate.

It was created in response to perceived problems with previous async functions like setInterval(), which for example doesn't run at a frame rate optimized for the device, dropping frames in some cases. They also lacked some optimizations suited for animations, like stopping the execution if the tab isn't active or the animation is scrolled off the page, among other things.

The method takes as an argument a callback to be invoked before the repaint. This is the general pattern you’ll see it used in:

function draw() {
// Drawing code goes here

The idea is to define a function in which your animation is updated (e.g. your sprites are moved, score is updated, data is refreshed, or whatever). Then, you call it to start the process off. At the end of the function block you call requestAnimationFrame() with the function reference passed as the parameter, and this instructs the browser to call the function again on the next display repaint. This is then run continuously, as the code is calling requestAnimationFrame() recursively.

Note: If you want to perform some kind of simple constant DOM animation, CSS Animations are probably faster. They are calculated directly by the browser’s internal code, rather than JavaScript.

If, however, you are doing something more complex and involving objects that are not directly accessible inside the DOM (such as 2D Canvas API or WebGL objects), requestAnimationFrame() is the better option in most cases.

How fast does your animation run?

The smoothness of your animation is directly dependent on your animation’s frame rate and it is measured in frames per second (fps). The higher this number is, the smoother your animation will look, to a point.

Since most screens have a refresh rate of 60Hz, the fastest frame rate you can aim for is 60 frames per second (FPS) when working with web browsers. However, more frames means more processing, which can often cause stuttering and skipping — also known as dropping frames, or jank.

If you have a monitor with a 60Hz refresh rate and you want to achieve 60 FPS you have about 16.7 milliseconds (1000 / 60) to execute your animation code to render each frame. This is a reminder that you'll need to be mindful of the amount of code that you try to run during each pass through the animation loop.

requestAnimationFrame() always tries to get as close to this magic 60 FPS value as possible. Sometimes, it isn't possible — if you have a really complex animation and you are running it on a slow computer, your frame rate will be less. In all cases, requestAnimationFrame() will always do the best it can with what it has available.

How does requestAnimationFrame() differ from setInterval() and setTimeout() ?

Let’s talk a little bit more about how the requestAnimationFrame() method differs from the other methods used earlier. Looking at our code from above:

function draw() {
// Drawing code goes here

Let’s now see how to do the same thing using setInterval():

function draw() {
// Drawing code goes here
setInterval(draw, 17);

As we covered earlier, you don’t specify a time interval for requestAnimationFrame(). It just runs it as quickly and smoothly as possible in the current conditions. The browser also doesn't waste time running it if the animation is offscreen for some reason, etc.

setInterval(), on the other hand requires an interval to be specified. We arrived at our final value of 17 via the formula 1000 milliseconds / 60Hz, and then rounded it up. Rounding up is a good idea; if you rounded down, the browser might try to run the animation faster than 60 FPS, and it wouldn't make any difference to the animation's smoothness, anyway. As we said before, 60Hz is the standard refresh rate.

Including a timestamp

The actual callback passed to the requestAnimationFrame() function can be given a parameter, too: a timestamp value, that represents the time since the requestAnimationFrame() started running.

This is useful as it allows you to run things at specific times and at a constant pace, regardless of how fast or slow your device might be. The general pattern you’d use looks something like this:

let startTime = null;function draw(timestamp) {
if (!startTime) {
startTime = timestamp;
currentTime = timestamp - startTime; // Do something based on current time requestAnimationFrame(draw);

Browser support

requestAnimationFrame() is supported in more recent browsers than setInterval()/setTimeout(). Interestingly, it is available in Internet Explorer 10 and above.

So, unless you need to support older versions of IE, there is little reason to not use requestAnimationFrame().

A simple example

Enough with the theory! Let’s build your own personal requestAnimationFrame() example. You're going to create a simple "spinner animation"—the kind you might see displayed in an app when it is busy connecting to the server, etc.

Note: In a real world example, you should probably use CSS animations to run this kind of simple animation. However, this kind of example is very useful to demonstrate requestAnimationFrame() usage, and you'd be more likely to use this kind of technique when doing something more complex such as updating the display of a game on each frame.

  1. Put an empty <div> element inside the <body>, then add a ↻ character inside it. This circular arrow character will act as our spinner for this example.
  2. Apply the following CSS to the HTML template (in whatever way you prefer). This sets a red background on the page, sets the <body> height to 100% of the <html> height, and centers the <div> inside the <body>, horizontally and vertically.
  1. Insert the following JavaScript inside your <script> element. Here, you're storing a reference to the <div> inside a constant, setting a rotateCount variable to 0, setting an uninitialized variable that will later be used to contain a reference to the requestAnimationFrame() call, and setting a startTime variable to null, which will later be used to store the start time of the requestAnimationFrame().

Below the previous code, insert a draw() function that will be used to contain our animation code, which includes the timestamp parameter:

Inside draw(), add the following lines. They will define the start time if it is not defined already (this will only happen on the first loop iteration), and set the rotateCount to a value to rotate the spinner by (the current timestamp, take the starting timestamp, divided by three so it doesn't go too fast):

Below the previous line inside draw(), add the following block — this ensures that the value of rotateCount is between 0 and 359, by setting the value to its modulo of 360 (i.e. the remainder left over when the value is divided by 360) — so the circle animation can continue uninterrupted, at a sensible, low value. Note that this isn't strictly necessary, but it is easier to work with values of 0359 degrees than values like "128000 degrees".

Next, below the previous block add the following line to actually rotate the spinner:

At the very bottom inside the draw() function, insert the following line. This is the key to the whole operation — you are setting the variable defined earlier to an active requestAnimation() call, which takes the draw() function as its parameter. This starts the animation off, constantly running the draw() function at a rate as near 60 FPS as possible.

Below the draw() function definition, add a call to the draw() function to start the animation.

Clearing a requestAnimationFrame() call

Clearing a requestAnimationFrame() call can be done by calling the corresponding cancelAnimationFrame() method. (Note that the function name starts with "cancel", not "clear" as with the "set..." methods.)

Just pass it the value returned by the requestAnimationFrame() call to cancel, which you stored in the variable rAF:


Active learning: Starting and stopping our spinner

In this exercise, we’d like you to test out the cancelAnimationFrame() method by taking our previous example and updating it, adding an event listener to start and stop the spinner when the mouse is clicked anywhere on the page.

Some hints:

  • You’ll want to add a tracking variable to check whether the spinner is spinning or not, clearing the animation frame if it is, and calling it again if it isn’t.

Throttling a requestAnimationFrame() animation

One limitation of requestAnimationFrame() is that you can't choose your frame rate. This isn't a problem most of the time, as generally you want your animation to run as smoothly as possible. But what about when you want to create an old school, 8-bit-style animation?

This was a problem, for example, in the Monkey Island-inspired walking animation from our Drawing Graphics article:

In this example, you have to animate both the position of the character on the screen, and the sprite being shown. There are only 6 frames in the sprite’s animation. If you showed a different sprite frame for every frame displayed on the screen by requestAnimationFrame(), Guybrush would move his limbs too fast and the animation would look ridiculous. This example therefore throttles the rate at which the sprite cycles its frames using the following code:

if (posX % 13 === 0) {
if (sprite === 5) {
sprite = 0;
} else {

So the code only cycles the sprite once every 13 animation frames.

…Actually, it’s about every 6.5 frames, as we update posX (character's position on the screen) by two each frame:

if (posX > width/2) {
newStartPos = -( (width/2) + 102 );
posX = Math.ceil(newStartPos / 13) * 13;
} else {
posX += 2;

This is the code that calculates how to update the position in each animation frame.

The method you use to throttle your animation will depend on your particular code. For instance, in the earlier spinner example, you could make it appear to move slower by only increasing rotateCount by one on each frame, instead of two.

Active learning: a reaction game

For the final section of this article, you’ll create a 2-player reaction game. The game will have two players, one of whom controls the game using the A key, and the other with the L key.

When the Start button is pressed, a spinner like the one we saw earlier is displayed for a random amount of time between 5 and 10 seconds. After that time, a message will appear saying "PLAYERS GO!!" — once this happens, the first player to press their control button will win the game.

Let’s work through this:

  1. Inside the empty <script> element on your page, start by adding the following lines of code that define some constants and variables you'll need in the rest of the code:
  1. A reference to the spinner, so you can animate it.
  2. A reference to the <div> element that contains the spinner, used for showing and hiding it.
  3. A rotate count. This determines how much you want to show the spinner rotated on each frame of the animation.
  4. A null start time. This will be populated with a start time when the spinner starts spinning.
  5. An uninitialized variable to later store the requestAnimationFrame() call that animates the spinner.
  6. A reference to the Start button.
  7. A reference to the results paragraph.
  8. Next, below the previous lines of code, add the following function. It takes two numbers and returns a random number between the two. You’ll need this to generate a random timeout interval later on.

Now it is time to set up the initial state of the app when the page first loads. Add the following two lines, which hide the results paragraph and spinner container using display: none;

Next, define a reset() function, which sets the app back to the original state required to start the game again after it has been played. Add the following at the bottom of your code:

Okay, enough preparation! It’s time to make the game playable! Add the following block to your code. The start() function calls draw() to start the spinner spinning and display it in the UI, hides the Start button so you can't mess up the game by starting it multiple times concurrently, and runs a setTimeout() call that runs a setEndgame() function after a random interval between 5 and 10 seconds has passed. The following block also adds an event listener to your button to run the start() function when it is clicked.

Note: You’ll see this example is calling setTimeout() without storing the return value. (So, not let myTimeout = setTimeout(functionName, interval).)

This works just fine, as long as you don’t need to clear your interval/timeout at any point. If you do, you’ll need to save the returned identifier!

The net result of the previous code is that when the Start button is pressed, the spinner is shown and the players are made to wait a random amount of time before they are asked to press their button. This last part is handled by the setEndgame() function, which you'll define next.

Add the following function to your code next:

Stepping through this:

First, cancel the spinner animation with cancelAnimationFrame() (it is always good to clean up unneeded processes), and hide the spinner container.

Next, display the results paragraph and set its text content to “PLAYERS GO!!” to signal to the players that they can now press their button to win.

Attach a keydown event listener to the document. When any button is pressed down, the keyHandler() function is run.

Inside keyHandler(), the code includes the event object as a parameter (represented by e) — its key property contains the key that was just pressed, and you can use this to respond to specific key presses with specific actions.

Set the variable isOver to false, so we can track whether the correct keys were pressed for player 1 or 2 to win. We don't want the game ending when a wrong key was pressed.

Log e.key to the console, which is a useful way of finding out the key value of different keys you are pressing.

When e.key is "a", display a message to say that Player 1 won, and when e.key is "l", display a message to say Player 2 won. (Note: This will only work with lowercase a and l — if an uppercase A or L is submitted (the key plus Shift), it is counted as a different key!) If one of these keys was pressed, set isOver to true.

Only if isOver is true, remove the keydown event listener using removeEventListener() so that once the winning press has happened, no more keyboard input is possible to mess up the final game result. You also use setTimeout() to call reset() after 5 seconds — as explained earlier, this function resets the game back to its original state so that a new game can be started.

That’s it — you’re all done!



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