I've seen this in a few places
function fn() {
return +new Date;
}
And I can see that it is returning a timestamp rather than a date object, but I can't find any documentation on what the plus sign is doing.
Can anyone explain?
7 Answers
That's the + unary operator. It's equivalent to:
function(){ return Number(new Date); }
JavaScript is loosely typed, so it performs type coercion/conversion in certain circumstances:
http://blog.jeremymartin.name/2008/03/understanding-loose-typing-in.html
http://www.jibbering.com/faq/faq_notes/type_convert.html
Other examples:
>>> +new Date()
1224589625406
>>> +"3"
3
>>> +true
1
>>> 3 == "3"
true
If you remember, when you want to find the time difference between two dates, you simply do as follows:
var d1 = new Date("2000/01/01 00:00:00");
var d2 = new Date("2000/01/01 00:00:01"); //one second later
var t = d2 - d1; //will be 1000 (msec) = 1 sec
typeof t; // "number"
Now if you check type of d1-0, it is also a number:
t = new Date() - 0; //numeric value of Date: number of msec's since 1 Jan 1970.
typeof t; // "number"
That + will also convert the Date to Number:
typeof (+new Date()) //"number"
But note that 0 + new Date() will not be treated similarly! It will be concatenated as string:
0 + new Date() // "0Tue Oct 16 05:03:24 PDT 2018"
It does exactly the same thing as:
function(){ return 0+new Date; }
that has the same result as:
function(){ return new Date().getTime(); }
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