What does the plus sign do in '+new Date'

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?

That's the + unary operator. It's equivalent to:

function(){ return Number(new Date); }

See http://xkr.us/articles/javascript/unary-add and MDN.

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

Here is the specification regarding the "unary add" operator. Hope it helps...

A JavaScript date can be written as a string:

Thu Sep 10 2015 12:02:54 GMT+0530 (IST)

or as a number:

1441866774938

Dates written as numbers, specifies the number of milliseconds since January 1, 1970, 00:00:00.

Coming to your question it seams that by adding '+' after assignment operator '=' , converting Date to equal number value.

same can be achieve using Number() function, like Number(new Date());

var date = +new Date(); //same as 'var date =number(new Date());'

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 is a unary add operator and also used for explicit Number conversion, so when you call +new Date(), it tries to get the numeric value of that object using valueOf() like we get string from toString()

new Date().valueOf() == (+new Date)  // true

It does exactly the same thing as:

function(){ return 0+new Date; }

that has the same result as:

function(){ return new Date().getTime(); }