Intervals and Timeouts

Syntax#

• timeoutID = setTimeout(function() {}, milliseconds)
• intervalID = setInterval(function() {}, milliseconds)
• timeoutID = setTimeout(function() {}, milliseconds, parameter, parameter, …)
• intervalID = setInterval(function() {}, milliseconds, parameter, parameter, …)
• clearTimeout(timeoutID)
• clearInterval(intervalID)

Remarks#

If the delay is not specified, it defaults to 0 milliseconds. However, the actual delay will be longer than that; for example, the HTML5 spec specifies a minimum delay of 4 milliseconds.

Even when setTimeout is called with a delay of zero, the function that is called by setTimeout will be executed asynchronously.

Note that many operations like DOM manipulation are not necessarily completed even if you’ve made the operation and moved on to the next code sentence, so you shouldn’t assume they will run synchronously.

Using setTimeout(someFunc, 0) enqueues the execution of the someFunc function at the end of the current JavaScript engine’s call stack, so the function will be called after those operations completed.

It is possible to pass a string containing JavaScript code (setTimeout("some..code", 1000)) in place of the function (setTimeout(function(){some..code}, 1000)). If the code is placed in a string, it will be later parsed using eval(). String-style timeouts are not recommended for performance, clarity and sometimes security reasons, but you may see older code which uses this style. Passing functions has been supported since Netscape Navigator 4.0 and Internet Explorer 5.0.

Intervals

function waitFunc(){
    console.log("This will be logged every 5 seconds");
}

window.setInterval(waitFunc,5000);

Removing intervals

window.setInterval() returns an IntervalID, which can be used to stop that interval from continuing to run. To do this, store the return value of window.setInterval() in a variable and call clearInterval() with that variable as the only argument:

function waitFunc(){
    console.log("This will be logged every 5 seconds");
}    

var interval = window.setInterval(waitFunc,5000);

window.setTimeout(function(){
    clearInterval(interval);
},32000);

This will log This will be logged every 5 seconds every 5 seconds, but will stop it after 32 seconds. So it will log the message 6 times.

Removing timeouts

window.setTimout() returns a TimeoutID, which can be used to stop that timeout from running. To do this, store the return value of window.setTimeout() in a variable and call clearTimeout() with that variable as the only argument:

function waitFunc(){
    console.log("This will not be logged after 5 seconds");
}
function stopFunc(){
    clearTimeout(timeout);
}

var timeout = window.setTimeout(waitFunc,5000);
window.setTimeout(stopFunc,3000);

This will not log the message because the timer is stopped after 3 seconds.

Recursive setTimeout

To repeat a function indefinitely, setTimeout can be called recursively:

function repeatingFunc() {
    console.log("It's been 5 seconds. Execute the function again.");
    setTimeout(repeatingFunc, 5000);
}

setTimeout(repeatingFunc, 5000);

Unlike setInterval, this ensures that the function will execute even if the function’s running time is longer than the specified delay. However, it does not guarantee a regular interval between function executions. This behaviour also varies because an exception before the recursive call to setTimeout will prevent it from repeating again, while setInterval would repeat indefinitely regardless of exceptions.

setTimeout, order of operations, clearTimeout

setTimeout

• Executes a function, after waiting a specified number of milliseconds.
• used to delay the execution of a function.

Syntax : setTimeout(function, milliseconds) or window.setTimeout(function, milliseconds)

Example : This example outputs “hello” to the console after 1 second. The second parameter is in milliseconds, so 1000 = 1 sec, 250 = 0.25 sec, etc.

setTimeout(function() {
    console.log('hello');
}, 1000);

Problems with setTimeout

if you’re using the setTimeout method in a for loop :

for (i = 0; i < 3; ++i) {
  setTimeout(function(){
    console.log(i);
  }, 500);
}

This will output the value 3 three times, which is not correct.

Workaround of this problem :

for (i = 0; i < 3; ++i) {
  setTimeout(function(j){
    console.log(i);
  }(i), 1000);
}

It will output the value 0,1,2. Here, we’re passing the i into the function as a parameter(j).

Order of operations

Additionally though, due to the fact that Javascript is single threaded and uses a global event loop, setTimeout can be used to add an item to the end of the execution queue by calling setTimeout with zero delay. For example:

setTimeout(function() {
    console.log('world');
}, 0);

console.log('hello');

Will actually output:

hello
world

Also, zero milliseconds here does not mean the function inside the setTimeout will execute immediately. It will take slightly more than that depending upon the items to be executed remaining in the execution queue. This one is just pushed to the end of the queue.

Cancelling a timeout

clearTimeout() : stops the execution of the function specified in setTimeout()

Syntax : clearTimeout(timeoutVariable) or window.clearTimeout(timeoutVariable)

Example :

var timeout = setTimeout(function() {
    console.log('hello');
}, 1000);

clearTimeout(timeout); // The timeout will no longer be executed

Intervals

Standard

You don’t need to create the variable, but it’s a good practice as you can use that variable with clearInterval to stop the currently running interval.

var int = setInterval("doSomething()", 5000 ); /* 5 seconds */
var int = setInterval(doSomething, 5000 ); /* same thing, no quotes, no parens */

If you need to pass parameters to the doSomething function, you can pass them as additional parameters beyond the first two to setInterval.

Without overlapping

setInterval, as above, will run every 5 seconds (or whatever you set it to) no matter what. Even if the function doSomething takes long than 5 seconds to run. That can create issues. If you just want to make sure there is that pause in between runnings of doSomething, you can do this:

(function(){

   doSomething();

   setTimeout(arguments.callee, 5000);

})()