Javascript idiom: What does if (x === +x) do?

Question

Reading through the source code of underscore.js I stumbled upon the following line:

... if (obj.length === +obj.length) { ...

That's a bit confusing for me. What is actually being compared here? I believe it has something to do about detecting native arrays, but cannot figure out what's actually going on. What does the + do? Why use === instead of ==? And what are the performance benefits of this style?

Explosion Pills · Accepted Answer · 2013-02-14T16:37:05.493

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The + coerces the value to an Number (much like !! coerces it to a boolean).

if (x === +x)

...can be used to confirm that x itself contains an integer value. In this case it may be to make sure that the length property of obj is an integer and has not been overwritten by a string value, as that can screw up iteration if obj is treated as an array.

edited Feb 14 '13 at 16:37

answered Feb 14 '13 at 16:31

Explosion Pills

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There are no integers in JavaScript. – Salman A Feb 14 '13 at 16:34
`+'5.5'` is `5.5`, you are looking for _Number_. (& @ Salman, Integers will be introduced in ES6) – Paul S. Feb 14 '13 at 16:35
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To be precise `+` is a shortcut for `parseFloat` and not for `parseInt`. So it coerces the value to float (or *"Number"* type). – VisioN Feb 14 '13 at 16:35

Salman A · Answer 2 · 2013-02-14T16:36:58.800

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It is a silly (IMO) way of checking if obj.length is a Number. This is better:

typeof obj.length == "number"

edited Feb 14 '13 at 16:36

answered Feb 14 '13 at 16:30

Salman A

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Why is this silly? What is the preferred alternative? – Saintali Feb 14 '13 at 16:31
I don't think it's silly. Maybe it is even faster than `typeof` way. – VisioN Feb 14 '13 at 16:34
lazy? yes.. silly? nahhh... `Number(obj.length)`is so much more to type in ;) – itsid Feb 14 '13 at 16:35
@SalmanA Well, the same goes for [this wonderful discovery](http://stackoverflow.com/q/14772076/1249581). – VisioN Feb 14 '13 at 16:37
I am actually surprised by the performance: http://jsperf.com/expr-expr-vs-typeof-expr – Salman A Feb 14 '13 at 16:50
@SalmanA Yeah, and this is really weird. So the method is not that silly as it might look like. – VisioN Feb 14 '13 at 16:58

Jay · Answer 3 · 2013-02-15T21:13:03.253

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The + coheres what is on the right side to be a number.

In this case if length was not a property on the object undefined would be returned. + undefined will yield Nan and this evalutation be false.

If the string can be coheres-ed into a number then it will be.. e.g + '1' will yield 1 as a Number this is especially important when dealing with hex values in string form e.g. +'0x7070' yields 28784

edited Feb 15 '13 at 21:13

answered Feb 14 '13 at 16:33

Jay

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Interesting. I guess the value of `+"everthing but a number"` is also `NaN`? – Saintali Feb 14 '13 at 16:37
Updated my answer to clarify! If you need anything else let me know! – Jay Feb 15 '13 at 21:13

Javascript idiom: What does if (x === +x) do?

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