Javascript: what is "- 0" doing? - Stack Overflow

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This is purely an educational question.

I'm working on a new version of a web app that the pany 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?

This is purely an educational question.

I'm working on a new version of a web app that the pany 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?

• javascript

Share Improve this question Follow asked Mar 4, 2014 at 20:22 DanTheManDanTheMan 55622 gold badges77 silver badges2121 bronze badges 2

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

Add a ment  | 

3 Answers 3

Sorted by: Reset to default 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).

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"

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

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