Create Array and fill with index plus one in JavaScript

Question

I am trying to create an Array using new Array() and filling with index + 1 but somehow, though array is creating successfully but values are not filling correctly.

Code -

var aa = (new Array(4)).map((x, index) => {
    return index + 1;
});

console.log(aa);

Output - [undefined, undefined, undefined, undefined]

Expected - [1, 2, 3, 4]

Let me know what I am doing wrong here.

Answer 1

map only visits entries that actually exist, it skips gaps in sparse arrays.

There are various ways to do this:

1. You can use Array.from and its mapping callback:

   const as = Array.from(Array(4), (_, index) => index + 1);
   
   console.log(as);

2. You can use a simple for loop:

   const as = [];
   for (let index = 1; index <= 4; ++index) {
       as.push(index);
   }
   
   console.log(as);

3. You can use fill and then map:

   const as = Array(4).fill().map((_, index) => index + 1);
   
   console.log(as);

4. A variant of fill+map, you can use spread as Igor shows.

Answer 2

[...Array(4)].map((v, i) => i + 1)