Why undefined plus undefined equals to Nan in Javascript

Question

How to understand undefined and null much better

console.log(undefined + 5) //NaN

console.log(null + 5) //5

console.log(undefined + undefined) //NaN

@NickParsons. I got something like [this](https://stackblitz.com/edit/js-prftcs) — I don't know, Aug 01 '20 at 09:53
@Idon'tknow stackblitz shows `null` even when you `console.log(NaN)`, it seems like it shows `null` when it really should be showing `NaN` (see your browser console instead for more accurate results). — Nick Parsons, Aug 01 '20 at 09:56
@NickParsons yes It gives `NaN`. My entire question goes wrong. because of stackblitz.. Thanks.. — I don't know, Aug 01 '20 at 09:58

score 5 · Accepted Answer · answered Aug 01 '20 at 09:58

5

+ operator applies an implicit coercion to an integer to an operand when the other is a number

undefined is coerced to NaN, so console.log(undefined + 5) and console.log(undefined + undefined) output NaN

null is coerced to number 0, so console.log(null + 5) is equivalent to console.log(0 + 5). Therefore 5 is outputted

answered Aug 01 '20 at 09:58

Maxime Helen

1

Specified in section [9.3 ToNumber](http://es5.github.io/#x9.3) – Moritz Roessler Aug 01 '20 at 10:06

1 Answers1