花式数组扁平化

flat.js

const arr = [1, [2, [3], [4]]];
arr.flat(Infinity);

function _flat(arr) {
  return (
    Array.isArray(arr)
      ? arr.reduce((acc, cur) => [...acc, ..._flat(cur)], [])
      : [arr]
  )
}

function _flat2(arr) {
  return (
    arr.reduce((acc, curr) => {
      if(Array.isArray(curr)) {
        return [...acc, ..._flat2(curr)];
      }

      return [...acc, curr];
    }, [])
  )
}

function myFlat(arr, depth = 1) {
  return (
    depth > 0 
    ? arr.reduce((acc, curr) => {
        if(Array.isArray(curr)) {
          return [...acc, ...myFlat(curr, depth - 1)];
        }

        return [...acc, curr];
      }, [])
    : arr
  )
}

function flatWithStack(arr) {
  const stack = [...arr];
  const res = [];

  while(stack.length) {
    const curr = stack.pop();

    if (Array.isArray(curr)) {
      stack.push(...curr);
    } else {
      res.unshift(curr);
    }
  }

  return res;
}