array-flatten.js

var a = [1, {a: [2, [3]]}, 'four', [[5], [6]], [[7], 8, 9], 10];


function flatten(arr) {
  let idx = 0;
  
  while(idx < arr.length) {
    if(arr[idx] instanceof Array) {
      Array.prototype.splice.apply(arr, [idx, 1].concat(arr[idx]));
    }else {
      idx++;
    }
  }
}
                                         
                                         
flatten(a);
console.log(a);

function flattenArray(arr, flattened=[]) {
    for(let item of arr) {
        if(item instanceof Array) {
            flattenArray(item, flattened);
        }else {
            flattened.push(item);
        }
    }
    return flattened;
}