Array.prototype.flat()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.

實驗性質: 這是一個實驗中的功能
此功能在某些瀏覽器尚在開發中，請參考兼容表格以得到不同瀏覽器用的前輟。

flat() 函數以遞迴方式將特定深度的子陣列重新串接成為一新的陣列

嘗試一下

const arr1 = [0, 1, 2, [3, 4]];

console.log(arr1.flat());
// expected output: Array [0, 1, 2, 3, 4]

const arr2 = [0, 1, [2, [3, [4, 5]]]];

console.log(arr2.flat());
// expected output: Array [0, 1, 2, Array [3, Array [4, 5]]]

console.log(arr2.flat(2));
// expected output: Array [0, 1, 2, 3, Array [4, 5]]

console.log(arr2.flat(Infinity));
// expected output: Array [0, 1, 2, 3, 4, 5]

語法

var newArray = arr.flat([depth]);

參數

depth 選擇性: 指定巢狀陣列展開的深度。預設為 1。

回傳值

函數將會回傳一個由原先陣列的子陣列串接而成的新陣列。

範例

展開巢狀陣列

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

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

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

當遭遇空元素時

flat() 函數會自動清除陣列中空的元素

var arr4 = [1, 2, , 4, 5];
arr4.flat();
// [1, 2, 4, 5]

替代方案

`reduce` 與 `concat`

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

//展開單層陣列
arr1.reduce((acc, val) => acc.concat(val), []); // [1, 2, 3, 4]

//欲展開更深層的巢狀結構請使用reduce與concat的遞迴
function flattenDeep(arr1) {
  return arr1.reduce(
    (acc, val) =>
      Array.isArray(val) ? acc.concat(flattenDeep(val)) : acc.concat(val),
    [],
  );
}
flattenDeep(arr1); // [1, 2, 3, 1, 2, 3, 4, 2, 3, 4]

//使用stack來實作非遞迴的展開
var arr1 = [1, 2, 3, [1, 2, 3, 4, [2, 3, 4]]];
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();
}
flatten(arr1); // [1, 2, 3, 1, 2, 3, 4, 2, 3, 4]

// 递归版本的反嵌套
function flatten(array) {
  var flattend = [];
  (function flat(array) {
    array.forEach(function (el) {
      if (Array.isArray(el)) flat(el);
      else flattend.push(el);
    });
  })(array);
  return flattend;
}

規範

Specification
ECMAScript® 2026 Language Specification # sec-array.prototype.flat