20191015

• Array Flattening

1. es2019 method

   arr.flat(Infinity)

2. Using helper function

   let flatten = (arr) => {
   let result=[]
   let loop = (arr) => {
   for (let i = 0; i < arr.length; i++) {
   if (Array.isArray(arr[i])) {
   loop(arr[i]);
   }else{
   result.push(arr[i]);
   }
   }
   }
   loop(arr);
   return result;
   }

3. Without helper function

   let flatten(arr) => {
   let res = [];
   
   let n = arr.length;
   for (let i = 0; i < n; i++) {
   if (Array.isArray(arr[i])) {
   res = res.concat(flatten(arr[i]));
   }
   else {
   res.push(arr[i]);
   }
   }
   
   return res;
   }

4. Using some

   let flatten = (arr) => {
   //concat method flattens one level each time
   let res = [].concat(…arr);
   return res.some(Array.isArray) ? flatten(res) : res;
   }

5. Using reduce

   let flatten = (arr) =>{
   return arr.reduce(
   (pre,cur)=>pre.concat(Array.isArray(cur)?newArr(cur):cur)
   ,[])
   }

6. Using toString

   arr.toString().split(‘,’);

• Remove specified element from array

  let remove = (arr, item) => {
  for (let i = 0; i < arr.length; i++) {
  if (arr[i] === item) {
  arr.splice(i, 1);
  i–;
  }
  }
  return arr;
  }

• Promise Batch Execution