Coerce to number

Question

I have often seen the trick

after = +after;

to coerce the variable after to a number. Reading through the Node.JS source I fou

Answer 1

After a quick google to make sure my suspicions were true, I came to this conclusion. Using the + operator to convert to a number is faster, because no mathematical operations occur after type-casting, whereas using the *= approach means that after after is converted, it will be multiplied by 1.

Answer 2

Yes. Both Unary Operator + and Multiplicative Operators such as * (called from Compound Assignment op=) invoke the internal ToNumber algorithm.

You can even use a 3rd option by statically calling the Number constructor:

after = Number(after);

Answer 3

Yes, however note that it is only the unary + operator that does this. I.e. 10 + "10" will give you "1010".

A perhaps less error prone option is to use what asm.js does:

10 + ("10"|0)

Although on the down-side it does require the brackets. It should be the fastest option in any case (probably equal to unary +).

Answer 4

Note: In some instances after = after-0 invokes different behaviour than after = after+0. I've noticed it with dates.

This is tested in Chrome v39 only:

var date = new Date(2000,0,1);
date += date; //"Sat Jan 01 2000 00:00:00 GMT+0000 (GMT Standard Time)Sat Jan 01 2000 00:00:00 GMT+0000 (GMT Standard Time)"
var date2 = new Date(2000,0,1);
date2 + 0; //"Sat Jan 01 2000 00:00:00 GMT+0000 (GMT Standard Time)0"
date2 - 0; //946684800000
date2 * 1; //946684800000

I don't know what is defined in the JS spec, but with dates, because both the date and the number can be cast to a string, and the + operator works on a string, then Chrome goes with a string concatenation. Because the - operator has no string equivalent, it falls back to number comparison.

I've found this useful when coercing dates into numbers for comparisons