How To Schedule Timer-Based Callbacks With Example

Karthik Elumalai
Nov 16, 2017
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Introduction

In this article, we will look at how the JavaScript timer functions - settimeout, setinterval, and Cleartimeout -- are used to schedule and cancel timer-based Callbacks, with a simple example of stopwatch program.

Brief of settimeout

In simple words, it will call any function after the specified time (milliseconds). This specified time is also called a delay.
For example - if we specify the delay as 5000, then it will wait for 5 seconds and start to execute the function which was passed as a parameter.

Below is the syntax

var id = setTimeout(fn, delay)

First Parameter(fn) => Any function name

Second Parameter(delay) => specifies the no. of milliseconds to wait before it starts the execution of given function

Note
The settimeout function returns a unique ID which can be used to cancel the timer at any time using Cleartimeout which is discussed below.

Example - Implemented stopwatch functionality using settimeout

<html>
<head>
<script type="text/javascript">
window.onload = function() {
var startbutton = document.getElementById("btnstart");
var stopbutton = document.getElementById("btnstop");
var clearbutton = document.getElementById("btnclear");
var seconds = 0,
minutes = 0,
hours = 0,
currenttimervalue = 0;
function CalculateTimerPartsAndDisplay() {
//increment logic
seconds++;
if (seconds >= 60) {
seconds = 0;
minutes++;
if (minutes >= 60) {
minutes = 0;
hours++;
} //end of if(minutes>= 60)
} //end of if(seconds>= 60)
//display
document.getElementById("res").innerHTML = (hours ? (hours > 9 ? hours : "0" + hours) : "00") + ":" + (minutes ? (minutes > 9 ? minutes : "0" + minutes) : "00") + ":" + (seconds ? (seconds > 9 ? seconds : "0" + seconds) : "00")
//recursive
timerlogicintiator();
} //end of function CalculateTimerPartsAndDisplay()
function timerlogicintiator() {
currenttimervalue = setTimeout(CalculateTimerPartsAndDisplay, 1000);
} //end of function timerlogicintiator()
//intial call of timer logic
timerlogicintiator();
//call timer logic when startbutton clicks
startbutton.onclick = timerlogicintiator;
//stop the watch by using clearTimeout function
stopbutton.onclick = function() {
clearTimeout(currenttimervalue);
}
clearbutton.onclick = function() {
document.getElementById("res").innerHTML = "00:00:00";
seconds = 0, minutes = 0, hours = 0;
}
} //end of onload function
</script>
</head>
<body>
<h1 id="res">00:00:00
</h1>
<input type="button" value="start" id="btnstart" />
<input type="button" value="stop" id="btnstop" />
<input type="button" value="clear" id="btnclear" />
</body>
<html>

Output

Live Demo

Brief of Setinterval

It is similar to the settimeout function, but it repeats a call to the same function at a given time.

FYI as recap

Settimeout won’t repeat the call, it just calls a function only once.

Syntax of Setinterval

var id = setInterval(fn, delay);

First Parameter(fn) => Any function name

Second Parameter(delay) => specifies the no of milliseconds to wait between each execution.

Note
By using this advantage of setinterval, we can just avoid the recursive call which was implemented in the above settimeout program.

Example

This time, we implemented same stopwatch functionality also using setinterval to differentiate and understand easily.

<html>
<head>
<script type="text/javascript">
window.onload = function() {
var startbutton = document.getElementById("btnstart");
var stopbutton = document.getElementById("btnstop");
var clearbutton = document.getElementById("btnclear");
var seconds = 0,
minutes = 0,
hours = 0,
currenttimervalue = 0;
//start of button click logics
//call timer logic when startbutton clicks
startbutton.onclick = timerlogicintiator;
//stop the watch by using clearTimeout function
stopbutton.onclick = function() {
clearTimeout(currenttimervalue);
}
clearbutton.onclick = function() {
document.getElementById("res").innerHTML = "00:00:00";
seconds = 0, minutes = 0, hours = 0;
}
//end of button click logics
function CalculateTimerPartsAndDisplay() {
//increment logic
seconds++;
if (seconds >= 60) {
seconds = 0;
minutes++;
if (minutes >= 60) {
minutes = 0;
hours++;
} //end of if(minutes>= 60)
} //end of if(seconds>= 60)
//display
document.getElementById("res").innerHTML = (hours ? (hours > 9 ? hours : "0" + hours) : "00") + ":" + (minutes ? (minutes > 9 ? minutes : "0" + minutes) : "00") + ":" + (seconds ? (seconds > 9 ? seconds : "0" + seconds) : "00")
} //end of function CalculateTimerPartsAndDisplay()
function timerlogicintiator() {
currenttimervalue = setInterval(CalculateTimerPartsAndDisplay, 1000);
} //end of function timerlogicintiator()
//initial call of timer logic
timerlogicintiator();
} //end of onload function
</script>
</head>
<body>
<h1 id="res">00:00:00</h1> <input type="button" value="start" id="btnstart" /> <input type="button" value="stop" id="btnstop" /> <input type="button" value="clear" id="btnclear" /> </body>
<html>