Recently, I came across a requirement where I had to call a function repeatedly after specific time interval, like sending ajax call at every 10 seconds. Sure, best option seems as setInterval, but it blew up my face like a cracker :)

In order to understand why setInterval is evil we need to keep in mind a fact that javascript is essentially single threaded, meaning it will not perform more than one operation at a time.

In cases when functions takes longer than delay mentioned in setInterval (like ajax call, which might it prevent from completing on time), we will find that either functions have no breathing room or setInterval breaks it's rhythm.

    var fakeCallToServer = function() {
        setTimeout(function() {
            console.log('returning from server', new Date().toLocaleTimeString());
        }, 4000);
    }



    setInterval(function(){ 

        let insideSetInterval = new Date().toLocaleTimeString();

        console.log('insideSetInterval', insideSetInterval);

        fakeCallToServer();
    }, 2000);

//insideSetInterval 14:13:47
//insideSetInterval 14:13:49
//insideSetInterval 14:13:51
//returning from server 14:13:51
//insideSetInterval 14:13:53
//returning from server 14:13:53 
//insideSetInterval 14:13:55
//returning from server 14:13:55

Try above code snippets in your console

As you can see from printed console.log statement that setInterval keeps on sending ajax calls relentlessly without caring previous call has returned or not.
This can queue up a lot of requests at once on the server.

Now, let's try a synchronous operation in setInterval:

var counter = 0;

var fakeTimeIntensiveOperation = function() {

    for(var i =0; i< 50000000; i++) {
        document.getElementById('random');
    }

    let insideTimeTakingFunction  = new Date().toLocaleTimeString();

    console.log('insideTimeTakingFunction', insideTimeTakingFunction);
}



var timer = setInterval(function(){ 

    let insideSetInterval = new Date().toLocaleTimeString();

    console.log('insideSetInterval', insideSetInterval);

    counter++;
    if(counter == 1){
        fakeTimeIntensiveOperation();
    }

    if (counter >= 5) {
       clearInterval(timer);
    }
}, 1000);

//insideSetInterval 13:50:53
//insideTimeTakingFunction 13:50:55
//insideSetInterval 13:50:55 <---- not called after 1s
//insideSetInterval 13:50:56
//insideSetInterval 13:50:57
//insideSetInterval 13:50:58

We see here when setInterval encounters time intensive operation, it does either of two things, a) try to get on track or b) create new rhythm. Here on chrome it creates a new rhythm.

Conclusion

In case of asynchronous operations, setTimeInterval will create long queue of requests which will be very counterproductive.
In case of time intensive synchronous operations, setTimeInterval may break the rhythm.
Also, if any error occurs in setInterval code block, it will not stop execution but keeps on running faulty code. Not to mention they need a clearInterval function to stop it.
Alternatively, you can use setTimeout recursively in case of time sensitive operations.