CODEWITHSUNDEEP
javascript
Martin
Martin

Reputation: 24308

Javascript: Changing a positive number to a negative?

I have been trying to do the following - change a positive to a negative number.

It appears there are a number of ways to do this. There is the standard

x *= -1

or just placing a negative sign in front the variable i.e if x = 5, then -x is equal to -5.

This seems a great shorthand but wanted to know what the difference is, I can't find any documentation regarding this shorthand on MDN.

I assume there are other ways too.

Probably a basic question but it is annoying not understanding this apparent shorthand.

Any ideas ?

Upvotes: 0

Views: 8612

Answers (1)

nikjohn
nikjohn

Reputation: 21852

Unary operators in Javascript are basically shorthand functions. You can find the documentation for the Unary (-) here

The - takes in one argument. The number you pass to it. Under the hood, I'm guessing it multiplies it by -1 and returns the product. The function could be written along the lines of:

function -(arg) {
  return arg * -1;
}

This is conjecture though. Will need to go through V8's codebase to know for sure.

Update:

So from further research I figure that it's instead of multiplication by -1, it could be a simple sign change. I referred to V8's implementation but that proved to be a dead end because I suck at C++, but upon checking ECMA's specs and the IEEE 754 specs defined here in Steve Hollasch's wonderful blog, I am leaning towards a inversion of the sign bit. All Javascript numbers are 64 Bit IEEE 754 FLoating Points, they can be represented like so:

SEEEEEEE EEEEMMMM MMMMMMMM MMMMMMMM MMMMMMMM MMMMMMMM MMMMMMMM MMMMMMMM

Where S is the sign bit. So it looks like the Unary - just flips the sign bit.

Upvotes: 6

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