Checking whether clearInterval has been called?

Given this code:

bob = setInterval(function, 1000);
clearInterval(bob);

Is there now a way to know if that interval has been cleared?

Currently, I keep track of this myself, by unsetting 'bob', but I'm curious if my extra line of code is unnecessary:

clearInterval(bob);
bob = null;
if (!bob) itIsCleared();

Thanks!

回答1:

The return value of setInterval is just a unique id you use to pass back to clearInterval. It's not a structured object with any additional information, nor does it get set to null when you call clearTimeout.

回答2:

bob only contains an id of the interval used to clear it. When you call clearInterval, it gets the interval associated with that id and clears it. The id isn't changed at all.

see here for demonstration

example:

<html>
<head>
<title>Javascript clearInterval</title>
</head>
<body onload="startInterval();">

<center>
    <div id="myTime"></div>

    <input type="button" value="start Interval" onclick="startInterval();" />

    <input type="button" value="stop Interval" onclick="stopInterval();" />
</center>

<script language="javascript">

var interval;

function startInterval()
{
    // setInterval of 1000 milliseconds i.e. 1 second
    // to recall the startTime() method again n again
    interval = setInterval("startTime();", 1000);
}

function startTime()
{
    // Date object to get current time
    var timeFormat = new Date();

    // set the current time into the HTML div object.
    document.getElementById('myTime').innerHTML = timeFormat.toLocaleTimeString();
}

function stopInterval()   //***********IMPORTANT FUNC******************
{
    // clearInterval to stop the setInterval event
    alert(interval);  
    clearInterval(1);

}

</script> 

</body>
</html>

This will show you the interval's id (returned by setInterval earlier). If you know the interval's id is 1, you can just use clearInterval(1) to clear the interval. So your way of using setting bob to null is a good way of doing it. Just be sure that !bob doesn't return true if the bob happens to be 0. :D