Flattern

Flattern.js

function flattern(arr) {

    const result = [];
    const stack = [arr];
    let temp;
    let length;
    let value;
    let i;
 
    while(stack.length !== 0){
      temp = stack.pop();
      length = temp.length;
      
      for (i = 0; i < length; i++) {
         value = temp[i];
         if (Array.isArray(value)) {
            stack.unshift(value);
            stack.unshift(temp.slice(i + 1));
            break;
         }
         result.push(value);
      }
    }

    return result;
}