Effect of x = +x in JavaScript [duplicate]

Question

Going though the asm.js documentation I've observed this strange (to me at least, quite new to JS) snippet all over the sample code:

function test(x) {
    x = +x; // THIS
    ...
    return +(x*y);
}

What is the purpose of the + on the first line?

Answer 1

Its simply used for casting a value with another type to number. Additonally it will return NaN if the value after that + symbol could not get converted into a number.

FIDDLE

From the book Javascript and Jquery - The Missing Maunal

var numOfShoes = '2';
var numOfSocks = 4;
var totalItems = +numOfShoes + numOfSocks;

Adding a + sign before a variable (make sure there’s no space between the two) tells the JavaScript interpreter to try to convert the string to a number value—if the string only contains numbers like “2”, you’ll end up with the string converted to a number. In this example, you end up with 6 (2 + 4). Another technique is to use the Number() command like this:

var numOfShoes = '2';
var numOfSocks = 4;
var totalItems = Number(numOfShoes) + numOfSocks;

Number() converts a string to a number if possible. (If the string is just letters and not numbers, you get the NaN value to indicate that you can’t turn letters into a number.)

Answer 2

Perhaps I am reading this wrong but from the specs http://asmjs.org/spec/latest/#parameter-type-annotations

is that casting it as a double?