JS Callbacks & Schedulers

 

1. Callback function

A Callback is a function that is passed as an argument to another function.

1.1 Passing a function as an argument

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function displayGreeting(displayName) {

console.log("Hello");

displayName();

console.log("Good Morning!");

}

displayGreeting(function() {

console.log("Rahul");

});

JAVASCRIPT

1.2 Passing a function name as an argument

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function displayGreeting(displayName) {

console.log("Hello");

displayName();

console.log("Good Morning!");

}

function displayRahul() {

console.log("Rahul");

}

displayGreeting(displayRahul);

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1.3 Passing a function expression as an argument

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function displayGreeting(displayName) {

console.log("Hello");

displayName();

console.log("Good Morning!");

}

let displayRam = function() {

console.log("Ram");

};

displayGreeting(displayRam);

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2. Schedulers

The Schedulers are used to schedule the execution of a callback function.

There are different scheduler methods.

• setInterval()
• clearInterval()
• setTimeout()
• clearTimeout()
  , etc.

2.1 setInterval()

The

setInterval()

method allows us to run a function at the specified interval of time repeatedly.

Syntax:

setInterval(function, delay);

function - a callback function that is called repeatedly at the specified interval of time (

delay

). delay - time in milliseconds. (1 second = 1000 milliseconds)

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let counter = 0;

setInterval(function() {

console.log(counter);

counter = counter+1;

}, 1000);

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In the

setInterval()

method, the callback function repeatedly executes until the browser tab is closed or the scheduler is cancelled.

When we call the

setInterval()

method, it returns a unique id. This unique Id is used to cancel the callback function execution.

2.2 clearInterval()

The

clearInterval()

method cancels a schedule previously set up by calling

setInterval()

.

To execute

clearInterval()

method, we need to pass the uniqueId returned by

setInterval()

as an argument.

Syntax:

clearInterval(uniqueId);

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let counter = 0;

let uniqueId = setInterval(function() {

console.log(counter);

counter = counter+1;

}, 1000);

clearInterval(uniqueId);

JAVASCRIPT

Try out the

setInterval()

and

clearInterval()

methods and check the output in the below Code Playground console.

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2.3 setTimeout()

The

setTimeout()

method executes a function after the specified time.

Syntax:

setTimeout(function, delay);

function - a callback function that is called after the specified time (

delay

). delay - time in milliseconds.

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let counter = 0;

setTimeout(function() {

console.log(counter);

counter = counter + 1;

}, 1000);

JAVASCRIPT

2.4 clearTimeout()

We can cancel the

setTimeout()

before it executes the callback function using the

clearTimeout()

method.

To execute

clearTimeout()

, we need to pass the uniqueId returned by

setTimeout()

as an argument.

Syntax:

clearTimeout(uniqueId);

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let counter = 0;

let uniqueId = setTimeout(function() {

console.log(counter);

counter = counter+1;

}, 1000);

clearTimeout(uniqueId);

JAVASCRIPT

Try out the

setTimeout()

and

clearTimeout()

methods and check the output in the below Code Playground console.

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