Why is "string" - "string" NaN?

Question

This is something like an infinite loop for me.

var sM = "Hello" - "World";
console.log(sM) && console.log(typeof sM);

I understand why string - string outputs NaN, but then "Hello" - "World" typeof is a number.

It means subtracting string with another string gives you a type of a number.

Where is the logic in that?

[jQuery.isNumeric](https://api.jquery.com/jQuery.isNumeric/) does return false for NaN values. typeof only returns the internal storage mechanism for that particular variable type. In this case its stored as a number, specifically a float stored according to IEEE 754 to correspond to NaN — apokryfos, May 18 '16 at 13:55

score 9 · Accepted Answer · edited May 23 '17 at 11:44

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why string - string outputs NaN

Because subtraction only deals in numbers, so it converts both sides to numbers and gets Not A Number.

but then "Hello" - "World" typeof is a number.

NaN has the type Number. This is standard in computing.

edited May 23 '17 at 11:44

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answered May 18 '16 at 13:39

Quentin

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Oh, so when you subtract two strings because of mathematical operation, the type changes, but since you of course cannot subtract strings with another strings it outputs as "Not a Number". – super11 May 18 '16 at 13:43
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Depends on the strings. `"12" - "10" === 2` – Quentin May 18 '16 at 13:44
So only if we use words-only in a strings? – super11 May 18 '16 at 13:45
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@Super11 only if `Number(yourString)` is `NaN` – XCS May 18 '16 at 13:51

score 2 · Answer 2 · edited May 23 '17 at 12:08

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Because typeof NaN is number.

It makes sense that you can't subtract 2 strings, so their subtraction is not a number (NaN).

What probably doesn't make as much sense is why NaN (not a number) is of type number. :)

See this answer for more details about the part that makes less sense.

edited May 23 '17 at 12:08

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answered May 18 '16 at 13:38

XCS

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Also because adding a mathematical operator `-` converts whatever follows into a number? – fnune May 18 '16 at 13:39

score 1 · Answer 3 · answered May 18 '16 at 14:06

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Actually this is just one between many cases where javascripts operations produce results that may be unexpected to the newcomer. This happens because of implicit type coersion. Which is a type change in values that occurs automatically in certain operations where different types are involved or expected. These cases are thoroughly described in this chapter from one of the You Don't Know Js free books, here:

https://github.com/getify/You-Dont-Know-JS/blob/master/types%20&%20grammar/ch4.md

I definitively recommend this book series to every JS developer.

answered May 18 '16 at 14:06

Nelson Teixeira

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Thanks for sharing, up voted. – super11 May 18 '16 at 14:07
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this and many other JS resources are in SO javascript tag info, here: http://stackoverflow.com/tags/javascript/info – Nelson Teixeira May 18 '16 at 14:08

Why is "string" - "string" NaN?

3 Answers3