This is my array of arrays:

arr_1 = [1,2,3]
arr_2 = [4,5,6]
arr_3 = [7,8,9]

arr = [arr_1, arr_2, arr_3]
arr = [[1,2,3], [4,5,6], [7,8,9]]

What I want to do is push all elements like so that the final array is like the following and insert another element at the beginning of my array:

arr = [[i,1,2], [3,4,5], [6,7,8], [9]]

All sub-arrays must not be more than 3 elements.

Thanks for your help.

This is my array of arrays:

arr_1 = [1,2,3]
arr_2 = [4,5,6]
arr_3 = [7,8,9]

arr = [arr_1, arr_2, arr_3]
arr = [[1,2,3], [4,5,6], [7,8,9]]

What I want to do is push all elements like so that the final array is like the following and insert another element at the beginning of my array:

arr = [[i,1,2], [3,4,5], [6,7,8], [9]]

All sub-arrays must not be more than 3 elements.

Thanks for your help.

• javascript
• arrays

Share Improve this question Follow asked Mar 26, 2018 at 16:03 HugoHugo 2,20466 gold badges2828 silver badges5252 bronze badges 0

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7 Answers 7

Sorted by: Reset to default 4

You could visit all inner arrays and unshift the leftover values from the previous loop.

var array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]],
    chunk = 3,
    item = 'x',
    i = 0,
    temp = [item];
    
while (i < array.length) {
    array[i].unshift(...temp);
    temp = array[i].splice(chunk, array[i].length - chunk);
    i++;
}
if (temp.length) {
    array.push(temp);
}

console.log(array.map(a => a.join(' ')));

You can use the function reduce

var arr = [[1,2,3], [4,5,6], [7,8,9]],
    newElem = "newOne",
    all = [newElem, ...arr.reduce((a, c) => [...a, ...c], [])], // All together
    // Build the desired output asking for the result of:
    // element mod 3 === 0
    result = all.reduce((a, c, i) => { 
      if (i % 3 === 0) a.push([c]);
      else a[a.length - 1].push(c);
      
      return a;
    }, []);
    
console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

You could move on each iteration last element from previous array to next one and if the last sub-array has more then 3 elements then remove the last one and add it to new array.

let arr_1 = [1, 2, 3],
  arr_2 = [4, 5, 6],
  arr_3 = [7, 8, 9],
  arr = [arr_1, arr_2, arr_3]

setInterval(function() {
  const last = arr.length - 1;
  const newElement = parseInt(Math.random() * 30)

  arr.forEach((a, i) => {
    if(i == 0) a.unshift(newElement);
    if(arr[i + 1]) arr[i + 1].unshift(a.pop())
    else if(arr[last].length > 3) arr[last + 1] = [arr[last].pop()]
  })

  console.log(JSON.stringify(arr))
}, 1000)

You can do this quite succinctly with a simple unravel/ravel. It easy to adjust group size too.

let arr = [ [1,2,3], [4,5,6], [7,8,9]]
let newEl = 0
let groupSize = 3

var newArr = [];
var unravelled = arr.reduce((a, c) => a.concat(c), [newEl])
while(unravelled.length) newArr.push(unravelled.splice(0,groupSize));

console.log(newArr)

arr_1 = [1, 2, 3]
arr_2 = [4, 5, 6]
arr_3 = [7, 8, 9]
arr = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]

function reassignChunk(x) {
  // create an array which will be used to store unwrapped values 
  var newArray = [];
  arr.forEach(function(elem) {
    newArray.push(...elem); //using spread operator to unwrap values
  });
  newArray.unshift(x, limit)
  var modArray = [];
  var m, j, temparray, chunk = 3;
  for (m = 0; m < newArray.length; m = m + limit) {
    // creatinging new array using slice
    modArray.push(newArray.slice(m, m + limit));
  }
  console.log(modArray)
}

reassignChunk(13, 3)

arr = [[i,1,2], [3,4,5], [6,7,8], [9]]

Assuming all your elements are numbers, you can do it like this:

1. Prepend i to the array
2. Flatten the array
3. Convert the array to a ma-separated string
4. Split the string into chunks of at most 3 numeric substrings (2 mas)
5. Convert the chunks back into arrays of numbers

const arr_1 = [1,2,3];
const arr_2 = [4,5,6];
const arr_3 = [7,8,9];

const i = 42;

const result = [i,...arr_1,...arr_2,...arr_3].join()
  .match(/(?:[^,]+(,|$)){1,2}[^,]*/g).map( x => x.split(',').map(Number) )
;

console.log( result );

You may do your 2d unshifting simply as follows;

var arr_1 = [1,2,3],
    arr_2 = [4,5,6],
    arr_3 = [7,8,9],
    arr   = [arr_1, arr_2, arr_3],
    us2d  = (a2d,...is) => is.concat(...a2d)
                             .reduce((r,e,i) => (i%3 ? r[r.length-1].push(e)
                                                     : r.push([e]), r), []);
console.log(JSON.stringify(us2d(arr,0)));
console.log(JSON.stringify(us2d(arr,-2,-1,0)));