jQuery .now() not unique

问题:

I have a jsfiddle.

obj = {};
obj[0] = 'hej';
obj[1] = 'hopp';

$.each( obj, function( key, value ) {
    console.log($.now());
});

$.now() is the same number in the console.

How can I generate a unique number?

回答1:

You could have a counter which increments for each object. You could then add this counter to $.now() to make sure it's unique:

obj = {};
obj[0] = 'hej';
obj[1] = 'hopp';
counter=1;

$.each( obj, function( key, value ) {
    console.log($.now() + counter);
    counter+=1;
});

回答2:

jQuery.now() isn't supposed to give you a unique number, just a " a number representing the current time" (http://api.jquery.com/jQuery.now/). More specifically "the number of milliseconds since 1 January 1970 00:00:00 UTC" (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getTime).

The simplest way to generate a unique identifier depends on the scope the identifier has to be unique in.

1: If the identifier just has to be unique within the scope of the function, a counter as suggested by SteveP would probably do the job best.

2: If the identifier needs to be unique within the scope of your application - and you do not have the possibility to keep track of the assigned identifiers (as in 1) - your job is more difficult. In real-live applications such a scenario could be tackled by combining the timestamp with a random number, along the lines of

var identifier = $.now() + "_" + Math.round(Math.random() * 1e10)

This approach doesn't guarantee for true uniqueness of identifier though since Math.round(Math.random() * 1e10) could return the same value twice in one millisecond. Depending on how often you generate a string like this, it will be still pretty unlikely you end up with the same string twice.

3: If you however aim for "uniqueness across space and time", you could get inspiration from http://www.ietf.org/rfc/rfc4122.txt

It should be noted that pure Javascript implementations like Create GUID / UUID in JavaScript? only work with random numbers as well and will thus not generate truly unique identifiers. For a truly unique identifier across space and time a truly unique element such as a mac-address has to be used as part of the identifier.

回答3:

Quote from jquery api:

The $.now() method is a shorthand for the number returned by the expression (new Date).getTime().

So you basically use new Date().getTime(). However getTime() isn't accurate... Interresting quote from this article:

Effectively these browsers are only updating their internal getTime representations every 15 milliseconds. This means that if you attempt to query for an updated time it’ll always be rounded down to the last time the timer was updated (which, on average, will have been about 7.5 milliseconds ago).

In short: getTime() isn't that accurate (in some browsers).

When you need a random number (as you do), use:

var randomVal = Math.random(); // e.g. 0.6137274974025786
var randomValI = Math.round(Math.random()*100000000000000000)  // e.g. 6137274974025786

When you need a more formal 'guid', use something like this:

function createGUID() {
    function S4() {
        return (((1+Math.random())*0x10000)|0).toString(16).substring(1); 
    }

    // then to call it, plus stitch in '4' in the third group
    return (S4() + S4() + "-" + S4() + "-4" + S4().substr(0,3) + "-" + S4() + "-" + S4() + S4() + S4()).toLowerCase();
}
var guid = createGUID();

回答4:

Of course $.now just display the current timestamp. to have a random number use Math.random()