Performant ES6 array of arrays flatten.

flatten_array.js

let arr = [[1,2,[3]], 4];
const flatten = arr => {
  let i = 0;
  while (i < arr.length) {
    if (Array.isArray(arr[i])) {
      arr.splice(i, 1, ...arr[i])
    } else {
      i++
    }
  }
  return arr
}
console.log(flatten(arr)); // [1, 2, 3, 4]