Javascript insert element at nth index in array, insertion elements from another array

Question

I have 2 Javascript arrays:

['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12']
['a', 'b', 'c']

I need to insert elements of second array into the first array every 4th or nth index of the first array, resulting in:

['a', '1', '2', '3', '4', 'b', '5', '6', '7', '8', 'c', '9', '10', '11', '12']

n needs to be a parameter, so I can move n to the 5th location, or 3rd location when needed.

Any solutions appreciated. ES6 ones would be great! Thank you in advance.

yeah, you'll want to use array splice method in some sort of loop — Jaromanda X, Apr 22 '19 at 13:35
Hi Mohamed, nothing yet. I've looked at splice, concat, map, and reduce examples, but my skill level is not sufficient to solve this problem and I was not able to find a similar question. — rew, Apr 22 '19 at 13:40
@jonas-wilms this question is sufficiently different than the one you point to as a duplicate. — rew, Apr 22 '19 at 13:58
Please ping @Rajesh for that, you closed the question and can actually reopen it. He is a gold badge holder, Bob. — double-beep, Apr 22 '19 at 14:31

Jonathan Gagne · Answer 1 · 2019-04-22T14:06:44.600

Use a forEach with iterator as below:

let arr1 = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12'];
let arr2 = ['a', 'b', 'c'];

let index = 0;
arr2.forEach(function(v){
  arr1.splice(index, 0, v);
  index += 5;
});

console.log(arr1);

Now about the ES6, here is how I would write it:

let arr1 = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12'];
let arr2 = ['a', 'b', 'c'];

let index = 0;
Array.from(arr2).forEach((v)=>{
  arr1.splice(index, 0, v);
  index += 5;
});

console.log(arr1);

score 1 · Accepted Answer · answered Apr 22 '19 at 13:41

You can iterate through the smaller array using forEach use splice() to insert elements.

let arr1 = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12']
let arr2 = ['a', 'b', 'c']

function insertArr(arr1,arr2,n){
  arr1 = arr1.slice();
  arr2.forEach((a,i) => {
    arr1.splice(i*n+i,0,a);
  })
  return arr1;
}

console.log(insertArr(arr1,arr2,4))

score 1 · Answer 3 · answered Apr 22 '19 at 13:43

1

Try this

b.forEach((itm, indx) => a.splice(Math.ceil(a.length/b.length)*indx, 0, itm));

answered Apr 22 '19 at 13:43

Zeyad Etman

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Ori Drori · Answer 4 · 2019-04-22T15:43:03.723

Iterate the 2nd array with Array.flatMap(), take the respective sequence from the 1st array with Array.slice(), and combine it with the current element using spread. Use Array.concat() with slice to add leftover items if any.

const fn = (n, added, target) =>
  added.flatMap((el, i) => [el, ...target.slice(n * i, n * (i + 1))])
  .concat(target.slice(n * added.length))

const arr1 = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12']
const arr2 = ['a', 'b', 'c']

console.log(JSON.stringify(fn(3, arr2, arr1)));
console.log(JSON.stringify(fn(4, arr2, arr1)));
console.log(JSON.stringify(fn(5, arr2, arr1)));

The first test case {console.log(JSON.stringify(fn(3, arr2, arr1)));}, truncates arr1. — rew, Apr 22 '19 at 14:25

Javascript insert element at nth index in array, insertion elements from another array

4 Answers4