Array.prototype.flat()

js

const arr = [1, 2, [3, 4]];

// 単一レベルの配列にする
arr.flat();
// 次のものと同様
arr.reduce((acc, val) => acc.concat(val), []);
// [1, 2, 3, 4]

// または、分割代入の構文を使用して
const flattened = (arr) => [].concat(...arr);

js

const arr = [1, 2, [3, 4, [5, 6]]];

// reduce と concat の再帰によって深いレベルを平坦化することができる
function flatDeep(arr, d = 1) {
  if (!Array.isArray(arr)) {
    return arr;
  }
  return d > 0
    ? arr.reduce((acc, val) => acc.concat(flatDeep(val, d - 1)), [])
    : arr.slice();
}

flatDeep(arr, Infinity);
// [1, 2, 3, 4, 5, 6]

js

// 再帰を使わずにスタックを使用して平坦化
// note that depth control is hard/inefficient as we will need to tag EACH value with its own depth
// also possible w/o reversing on shift/unshift, but array OPs on the end tends to be faster
function flatten(input) {
  const stack = [...input];
  const res = [];
  while (stack.length) {
    // pop value from stack
    const next = stack.pop();
    if (Array.isArray(next)) {
      // push back array items, won't modify the original input
      stack.push(...next);
    } else {
      res.push(next);
    }
  }
  // reverse to restore input order
  return res.reverse();
}

const arr = [1, 2, [3, 4, [5, 6]]];
flatten(arr);
// [1, 2, 3, 4, 5, 6]

js

function* flatten(array, depth) {
  if (depth === undefined) {
    depth = 1;
  }

  for (const item of array) {
    if (Array.isArray(item) && depth > 0) {
      yield* flatten(item, depth - 1);
    } else {
      yield item;
    }
  }
}

const arr = [1, 2, [3, 4, [5, 6]]];
const flattened = [...flatten(arr, Infinity)];
// [1, 2, 3, 4, 5, 6]

js

const arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]

const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]

const arr3 = [1, 2, [3, 4, [5, 6]]];
arr3.flat(2);
// [1, 2, 3, 4, 5, 6]

const arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity);
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

js

const arr5 = [1, 2, , 4, 5];
arr5.flat();
// [1, 2, 4, 5]