How to push values into array using recursive function?

Question

I'm trying to push values into array by recursive function, but I get the error message "Uncaught TypeError: Array.push() is not a function", although the push() method is a regular array method. Where the problem could be?

let someArr = ['a', 'b', 'c'];
let someNum = 3;

let fillArr = (arr, num) => x = (num != 0) ? fillArr(arr.push(num), num -1) : arr;

console.log(fillArr(someArr, someNum))

// expected output: ['a', 'b', 'c', 3, 2, 1];
// actual output: Uncaught TypeError: arr.push is not a function

Array.push returns an int, and you're passing that int as argument for arr in your recursive call. — deceze
– deceze ♦, Commented Mar 3, 2021 at 11:22

customcommander · Accepted Answer · 2021-03-03 12:10:03Z

3

The comma operator is often overlooked; it could provide a nice escape hatch:

const fillArr =
  (arr, num) =>
    (num === 0
      ? arr
      : fillArr((arr.push(num), arr), num - 1));
//..............^^^^^^^^^^^^^^^^^^^^

fillArr(['a', 'b', 'c'], 3)
//=> ["a", "b", "c", 3, 2, 1]

However you shouldn't work on the original array with Array#push as it will mutate it (unless it was your intention!)

const arr = ['a', 'b', 'c'];
fillArr(arr, 3);
arr;
//=> ["a", "b", "c", 3, 2, 1] Oops!

Instead you should work with a new array:

const arr = ['a', 'b', 'c'];
const fillArr =
  (arr, num, nums = []) =>
    (num === 0
      ? arr.concat(nums)
      : fillArr(arr, num - 1, (nums.push(num), nums)));

fillArr(arr, 3);
//=> ["a", "b", "c", 3, 2, 1]
arr;
//=> ["a", "b", "c"]

edited Mar 3, 2021 at 12:10

answered Mar 3, 2021 at 11:57

customcommander

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Comments

Karthikeyan · Accepted Answer · 2021-03-03 12:18:44Z

1

What Mr.@deceze said is correct. If you still want to do that.. Do something like

let fillArr = (arr, num) => x = (num != 0) ? fillArr(arr, num -1, arr.push(num)) : arr;

fillArr will receive someArr got num pushed and the last argument will be ignored

answered Mar 3, 2021 at 12:18

Karthikeyan

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