CODEWITHSUNDEEP
javascript
DanTheMan
DanTheMan

Reputation: 556

Javascript: what is "- 0" doing?

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?

Upvotes: 9

Views: 212

Answers (3)

Claudiu
Claudiu

Reputation: 229361

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).

Upvotes: 19

Charles380
Charles380

Reputation: 1279

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

Upvotes: 1

BLSully
BLSully

Reputation: 5929

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"

Upvotes: 1

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