TypeError: (intermediate value).keys(...).map is not a function

Question

I am getting a strange error, could somebody help me fix this out?

const myArray = new Array(4)
    .keys()
    .map((v, i) => (
          console.log('print:', v, i)
      )
    )
console.log('myArray print: ', myArray);

I am getting this error:

TypeError: (intermediate value).keys(...).map is not a function

update:

i want to use the i to look up some stuff from another existing array.

const myArray = new Array(4)
    .keys()
    .map((v, i) => ({
      name: i
      })
    )
console.log('myArray print: ', myArray);

edit:

this question is not a duplicate, It is trying to iterate over a certain number and create a new object using the iterations.

the function keys when applied to the array returns an object, and the map function is not available for objects. — Daksh M., Dec 06 '17 at 10:13
Same issue and solution as the dupe target, except that `keys` is an iterator object instead of a HTML collection. — Cerbrus, Dec 06 '17 at 10:14
Reopened question since it is not a duplicate of the linked question. — Derek 朕會功夫, Dec 06 '17 at 10:22
@Derek朕會功夫: How is it not a duplicate? It's the ___same issue___: Array-like return value that isn't an array. It's got the ___same solution___. — Cerbrus, Dec 06 '17 at 10:33
@Cerbrus Not really. An iterator from an array with unindexed `undefine`s is fundamentally different than a live list. They might both yield the same error message, but they are different issues and require different answers. — Derek 朕會功夫, Dec 06 '17 at 10:35
@Derek朕會功夫: `Array.from(new Array(4).keys())` or `[...new Array(4),keys()]` works just fine. (From the dupe target) — Cerbrus, Dec 06 '17 at 10:40
@Cerbrus Different answers as in different explanation. It just so happens that `Array.from` accepts both an iterator and a live list because they are both array-like. A rule of thumb is that a question is a duplicate if you can copy and paste an answer from the linked question without having to modify and it remains a valid answer. — Derek 朕會功夫, Dec 06 '17 at 10:43
And that's exactly my point. It's about converting array-like objects to the arrays a users expected them to be in the first place. It may not be the best dupe target, but I'm sure there's one out there. — Cerbrus, Dec 06 '17 at 10:45
"they are both array-like" uhm no. Please don't say array or even something in that direction to an iterator. It is a different thing. The close-dupe here is not justified imo. — KarelG, Dec 06 '17 at 10:47

marvel308 · Answer 1 · 2017-12-06T11:14:15.677

4

Array.keys() returns an iterator Object which is an object and does not have a property map in it. You can convert it to do

const myArray = [...new Array(4).keys()].map((v, i) => ({
      name: i
      })
    )
console.log('myArray print: ', myArray);

edited Dec 06 '17 at 11:14

answered Dec 06 '17 at 10:13

marvel308

10,288
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Solution: Just pass it through `Array.from` – Derek 朕會功夫 Dec 06 '17 at 10:18
@Derek朕會功夫 can you please give an example? – moaningalways Dec 06 '17 at 10:18
@moaningalways `Array.from(something.keys()).map` – Derek 朕會功夫 Dec 06 '17 at 10:19
Or you can do `[...new Array(4).keys()].map((v, i) => v)` – marvel308 Dec 06 '17 at 10:21
@Derek朕會功夫 could you please provide an answer with a working example? thanks – moaningalways Dec 06 '17 at 10:24
@moaningalways I believe that comment is more than enough to get you started. – Derek 朕會功夫 Dec 06 '17 at 10:27
2

This answer does not deserve some upvotes because it only explains the problem. It however does not provide an answer to the OP's problem. – KarelG Dec 06 '17 at 10:50
1

@KarelG the original question was why does it not work which is answered – marvel308 Dec 06 '17 at 11:02

score 3 · Accepted Answer · answered Dec 06 '17 at 10:32

3

You can use Array.from to get the expected output:

var myArray = Array.from(Array(4), (value, index) => ({ name: index}));
console.log('myArray print: ', myArray);

answered Dec 06 '17 at 10:32

Faly

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TypeError: (intermediate value).keys(...).map is not a function

update:

edit:

2 Answers2