Usage of setInterval() function in JavaScript

Question

I don't understand why beneath code is not working.

function doSome(){
    console.log("Hi");
}

setInterval(doSome(), '1000');

what I expected :

// every second
Hi
Hi
Hi
...

but result

TypeError: Cannot read property 'call' of undefined
    at wrapper [as _onTimeout] (timers.js:275:18)
    at Timer.listOnTimeout (timers.js:92:15)

Okay, If I change setInterval(doSome(), '1000'); to setInterval(function(){doSome()}, '1000'); The code works finely. But I don't know what is different, and why I have to wrap the function like function(){...} Can you tell me some hints, Thanks...

setInterval(doSome(), '1000'); ==> setInterval(doSome, 1000); when passing function as reference, use only function name. Remove () of doSome, time should not be string — Tushar
– Tushar, Commented Dec 29, 2015 at 3:44
Remove the parenthesis ().. you are invoking the function. You need to pass a reference, therefore it should be setInterval(doSome, 1000); — Josh Crozier
– Josh Crozier, Commented Dec 29, 2015 at 3:44

Timothy Murphy · Accepted Answer · 2015-12-29 03:56:53Z

2

The problem with your code is setInteval expects the first argument to be a reference to a function, while doSome() will evaluate to some value, in this case null.

What you want is the following:

function doSome(){
  console.log("hi");
}
setInterval(doSome, 1000);

As you noted, you could also use an anonymous function (which returns a reference to that function), that calls the function you really want to execute. This is useful for calling a function that takes parameters.

function doMore(x, y){
  if (x < y)
    console.log("Hi");
  else
    console.log("Bye");
}
setInterval(function(){ doMore(0,1);}, 1000);

The second example is trivial but shows how you can call functions that require parameters using setInterval.

edited Dec 29, 2015 at 3:56

answered Dec 29, 2015 at 3:51

Timothy Murphy

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Bhojendra Rauniyar · Accepted Answer · 2015-12-29 03:48:47Z

1

When you don't use anonymous function ie. named function then you're calling it using the parentheses so remove it and will work fine:

setInterval(doSome, 1000);//You still don't need to wrap interval time within string

answered Dec 29, 2015 at 3:48

Bhojendra Rauniyar

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Dmitri Pavlutin · Accepted Answer · 2015-12-29 04:13:46Z

1

The problem is that setInterval requires a function as it's first argument.
The beauty of JavaScript is that you could even return a function from doSome:

function doSome(param){
    return function() { console.log(param); }
}

setInterval(doSome("Hi"), 1000);

But of course the correct answer is:

function doSome(){
    return console.log("Hi");
}

setInterval(doSome, 1000);

edited Dec 29, 2015 at 4:13

answered Dec 29, 2015 at 4:07

Dmitri Pavlutin

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