问题描述:

I'm learning about setTimeout vs setInterval and I have got setInterval to work but not setTimeout. for example this doesnt work:

<!DOCTYPE HTML>

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

<title>Brewery</title>

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.6.1/jquery.min.js"></script>

<script type="text/javascript">

function doNothing()

{

var t = "hello wolrd";

}

function poll(){

$.ajax({

//url: "http://192.168.0.11/"+Math.random(),

url:"line-ajax.htm",

contentType:"text/html",

success: 'doNothing()'

});

}

setTimeout(poll(),2000);

</script>

<link rel="stylesheet" type="text/css" href="http://www.highcharts.com/highslide/highslide.css" />

</head>

<body>

<div id="container" style="width: 1200px; height: 500px; margin: 0 auto"></div>

</body>

</html>

but if i change only this

setTimeout(poll(),2000);

to

setInterval(poll(),2000);

it will make an ajax request every 2 seconds...

what is going on?

---edit

i have also tried setTimeout(poll,2000); but that doesn't make the ajax request every 2 sec either.

网友答案:

"I'm trying to get setTimeout to call the 'poll' function every 2 seconds, it is not"

The .setTimeout() method calls the function you pass it exactly once, after the specified delay.

The .setInterval() method calls the function you pass it repeatedly, with the specified delay between each call.

From the update to your question you seem to think they both do the same thing. Please read the doco I've linked to.

Note that you have to pass a function reference (or a string to be eval'd), so say:

setInterval(poll, 2000);
// NOT
setInterval(poll(), 2000);

The latter should not work because it calls poll() immediately and passes its return value (undefined) to setInterval(), so I really can't understand why it worked for you.

网友答案:

Remove the () inside the setTimeout or setInterval. Treat the function name as a variable, and that's what you're passing.

网友答案:

As the parameter of setTimeout is eval'd you should try this:

setTimeout("poll()",2000);

Or if you like to work with anonymous functions better than with strings that get eval'd:

setTimeout(function() {
  poll();
},2000);

I personally like the latter more.

网友答案:

The first argument to setTimeout() should be a reference to the function to be called, not a function call with (). So try:

setTimeout(poll, 2000);
//-----------^^^
// No parentheses...

Likewise, in the $.ajax() call, the success function should be a function pointer, not a string:

$.ajax({
        //url: "http://192.168.0.11/"+Math.random(),
        url:"line-ajax.htm",
        contentType:"text/html",
        // Function pointer to doNothing
        success: doNothing
  });
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