Javascript: what is "- 0" doing?

Question

This is purely an educational question.

I'm working on a new version of a web app that the company I'm working for had made earlier. when re-writing the math, I came across this:

document.getElementById("estResidual").value-0;

Thinking there was no purpose for the "-0", I removed it. But when I tried the calculations, the numbers were waaayyyyyy off. Then I tried re-adding the "-0", and voila! everything worked nicely.

The Question: What did the "-0" do to change the value?

Note - when you use + 0 it might still look like a concatenation. There is no confusion with the minus. — Floris
– Floris, Commented Mar 4, 2014 at 20:24
@Floris - just re-read your comment and realized that too... so I deleted mine. :) — Steve
– Steve, Commented Mar 4, 2014 at 20:34

Cory Danielson · Accepted Answer · 2014-03-06 20:06:56Z

19

It's an (ab)use of JavaScript's soft typing behavior. In this case, it will convert a string to a float:

> "13"
"13"
> "13"-0
13
> "1.01"-0
1.01

Unary + will do the same:

> +"13"
13
> +"9.9"
9.9

Note that using + will instead convert the integer 0 into a string and concatenate it:

> "13"+0
"130"

This is all standardized. For explicit details on how these operators should behave, you can always check the ECMAScript Language Specification (e.g. addition, subtraction. unary plus).

edited Mar 6, 2014 at 20:06

Cory Danielson

14.5k4 gold badges47 silver badges52 bronze badges

answered Mar 4, 2014 at 20:23

Claudiu

231k174 gold badges507 silver badges702 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

DanTheMan Over a year ago

Why "abuse"? Certainly learning different ways to use a language is a beneficial thing, no? Or is this a practice that's best avoided due to the ease with which problems could occur with sloppy coding?

Claudiu Over a year ago

@DanTheGameMan: I personally don't like it because it's not obvious at all what the intent is. After all, you had to ask on stackoverflow what it does =). And then seeing that -0 works, you might expect +0 to work the same way, given the properties of arithmetic, but that gives you a totally different result! I'd recommend using parseInt() or parseFloat()

DanTheMan Over a year ago

parseFloat seems to work nicely, and it is indeed much clearer on what's happening

BLSully · Accepted Answer · 2014-03-04 20:24:51Z

1

The JS engine re-casts on the fly to try and make statements work. So JS will cast "23" to an integer 23 when you try to perform math on it, and likewise it will convert integer 23 to string "23" if you do something like:

var a = 23;
console.log(23 + "asdf");
//outputs "23asdf"

answered Mar 4, 2014 at 20:24

BLSully

5,9291 gold badge29 silver badges46 bronze badges

1 Comment

Claudiu Over a year ago

It's not really a static typing issue. A statically typed language could have this exact behavior, wherein the type of the expression <string> - <integer> would be <integer>.

Charles380 · Accepted Answer · 2014-03-04 20:29:35Z

1

That forces the type of object to become a number rather than a string.

answered Mar 4, 2014 at 20:29

Charles380

1,2799 silver badges19 bronze badges

1 Comment

Charles380 Over a year ago

well Claudiu's answer is way better than mine

Collectives™ on Stack Overflow

Javascript: what is "- 0" doing?

3 Answers 3

3 Comments

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related