Why does a "for ... in" loop—for (var/let/const A in B)—make A a string in vanilla ES6? [duplicate]

Question

This is in Chromium 78:

for (var i in [1,3,5]) console.log(i+1)

Now, I expected for (var i in [1,3,5]) console.log(i+1) to output 1, 2, 3, because i should be an index value. I know the MDN docs mention that the order may come out strangely in this case, but why the type conversion?

for...in returns a string key. Also: Why is using “for…in” with array iteration a bad idea? — adiga
– adiga, Commented Nov 13, 2019 at 13:37
Checkout the "Array iteration and for...in" section of the manual, you referenced. There is the explanation. — WuDo
– WuDo, Commented Nov 13, 2019 at 13:40
Also see note section for...in should not be used to iterate over an Array where the index order is important" — The_ehT
– The_ehT, Commented Nov 13, 2019 at 13:40
That's because an array is really just a special type of object and object keys are always strings. — ibrahim mahrir
– ibrahim mahrir, Commented Nov 13, 2019 at 13:43
The much harsher problem with for ... in is, that unlike many other methods (e.g. Object.keys), it iterates the prototype chain aswell. Therefore, e.g. Array.prototype["surprise!"] = 1; for (let p in []) console.log(p); may be "surprise!"'ing. — ASDFGerte
– ASDFGerte, Commented Nov 13, 2019 at 13:55

Christian Vincenzo Traina · Accepted Answer · 2019-11-13 13:44:02Z

5

i is not the index, i is the property key of the array object. Property keys are always strings.

answered Nov 13, 2019 at 13:44

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