如何使用JavaScript获得时间上的差异？

Question

im trying to get the current time timeNow in javascript and at the end of the script get timeEnd which is when the script finished executing for example:

newDate = new Date();
timeNow = newDate.getTime();

for(i=0; i < 5; i ++){

    console.log("printing" + i);
}

timeEnd = timeNow - newDate.getTime();

console.log(timeEnd);

i want to get the time it took to excute this script in seconds or milliseconds, but the result is 0. i don't see why, can you help me thanks.

Answer 1

The Date object does not keep ticking once it's constructed, it only represents a single instant in time.

You need to re-evaluate the time at the end of the script and then compare that value with the first one.

Note that on modern ES5 browsers you can use the "static" Date.now() function to get the current time. This is less expensive that constructing a whole new Date object. On older browsers you can emulate this function with this shim:

Date.now = Date.now || function() {
    return +(new Date);
};

At which point the required code becomes:

var start = Date.now();

// do your stuff here
...

var end = Date.now();

var elapsed = end - start;

FWIW, your simple loop reports 2ms when run on the Chrome console on my laptop.

Answer 2

This happens because you create only a single Date object. These don't change; you will have to do:

timeEnd = timeNow - new Date().getTime();

(note the blank between new and Date )

Answer 3

var t = new Date().getTime();
// code
console.log(new Date().getTime() - t);

Answer 4

因为我想不到，执行5次迭代所需的时间不到一毫秒。

Answer 5

you can use

starttime = Number(new Date());

endtime = Number(new Date());

Answer 6

I actually don't think it is iterating in less than an millisecond. It think the problem is that when you call newDate.getTime() repeatedly it returns the same value each time. The object, once you create it, is set at the time it was created. You'll need to create a new Date() object to compare the original against.

From my Firebug Console:

>>> d = new Date()
Date {Thu Aug 23 2012 10:45:52 GMT-0600 (Mountain Daylight Time)}
>>> d.getTime()
1345740352582
>>> d.getTime()
1345740352582
>>> d.getTime()
1345740352582
>>> d.getTime()
1345740352582

I can assure you that I didn't manually make the calls to d.getTime() in less than a millisecond.

Answer 7

Basically, Do this

newDate = new Date();
timeNow = newDate.getTime();

for(i=0; i < 50; i++){
    console.log("printing" + i);
}

futureDate = new Date()
timeEnd =  futureDate.getTime();
timeLength = timeEnd - timeNow;


console.log(timeLength);