why map a Array of `undefined` return a Array of `undefined`

Question

I wanna create a function(m ,n){}, return a array, it's length of m, each value is n

function createArray(m, n){
  let a = new Array(6);
  return a.map(item => n)
}
const z = createArray(5,6);
console.log(z)

I supposed z will be a array of 6 why z is a array of undefined?

You are creating an array of length 6 with undefined values. `new Array(6)` does that. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#Syntax — Saugat, Aug 05 '17 at 16:50
@A.Akram it works, can u instruct why my style is not working a little bit? — Liuuil, Aug 06 '17 at 02:19
@Liuuil, I'm not expert, but as I understand: `new Array(6)` has the same effect as `a=[]; a.length=6`. Which only sets the length without allocating real slots. it seems `map` iterate through "defined" slots not as you expect to iterate through 0 => array's length . — A.Akram, Aug 08 '17 at 11:40

sheplu · Answer 1 · 2017-08-06T02:35:24.607

0

z is indeed an array of 6, but 6 empty slot because your new Array does just create an empty array with 6 slots but no value

Pre ES6

let arr = new Array(10).fill('5');

edited Aug 06 '17 at 02:35

answered Aug 05 '17 at 16:50

sheplu

1 Answers1