Question

I’m reading through a jquery plugin, and find this interesting syntax:

'sth'+ +new Date

It creates a numeric string which the author used for a unique id: sth1237004731731

I’m curious what kind of syntax it is, and if there’s some reading materials about it? Thanks!

Answer 1

It’s using some side effects of JavaScript’s type coercion to build a unique identifier (probably for an element). The confusing part is the +new Date. Here, new Date (or new Date()) returns a Date object. But putting a + or a - in front of this forces the JS interpreter to coerce this value to a Number object, and the way JS does Date > Number is by returning the timestamp (or getTime()).

So this code could be expressed differently like this:

var date = new Date(), // "Mon May 14 2012 10:03:58 GMT-0400 (EST)"
    timestamp = date.getTime(), // 1337004238612
    identifierString = "sth" + timestamp.toString();

You might reasonably claim that there’s no need to be so verbose, so I personally would probably have written this as:

var identifier = "sth" + (new Date()).getTime();

However, please avoid coding things like your example if you ever expect someone might have to maintain your code. If it stopped you in your tracks, it probably will stop a lot of people. Coding style is not merely about expressing intent to the interpreter, but expressing intent to human developers. If the code works in the browser but fails with a syntax error in most experienced developers’ heads, you’ve done it wrong, plain and simple.