Array.prototype.flat()

flat() 方法会按照一个可指定的深度递归遍历数组,并将所有元素与遍历到的子数组中的元素合并为一个新数组返回。

语法

var newArray = arr.flat([depth])

参数

depth 可选
指定要提取嵌套数组的结构深度,默认值为 1。

返回值

一个包含将数组与子数组中所有元素的新数组。

示例

扁平化嵌套数组

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]

//使用 Infinity,可展开任意深度的嵌套数组
var arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity);
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

扁平化与数组空项

flat() 方法会移除数组中的空项:

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

替代方案

使用 reduceconcat

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

// 展开一层数组
arr.flat();
// 等效于
arr.reduce((acc, val) => acc.concat(val), []);
// [1, 2, 3, 4]

// 使用扩展运算符 ...
const flattened = arr => [].concat(...arr);

reduce + concat + isArray + recursivity

// 使用 reduce、concat 和递归展开无限多层嵌套的数组
var arr1 = [1,2,3,[1,2,3,4, [2,3,4]]];

function flatDeep(arr, d = 1) {
   return d > 0 ? arr.reduce((acc, val) => acc.concat(Array.isArray(val) ? flatDeep(val, d - 1) : val), [])
                : arr.slice();
};

flattenDeep(arr1, Infinity);
// [1, 2, 3, 1, 2, 3, 4, 2, 3, 4]

使用堆栈stack

// 注意:深度的控制比较低效,因为需要检查每一个值的深度
// also possible w/o reversing on shift/unshift, but array OPs on the end tends to be faster
var arr1 = [1,2,3,[1,2,3,4, [2,3,4]]];
function flatten(input) {
  const stack = [...input];
  const res = [];
  while (stack.length) {
    // 使用 pop 从 stack 中取出并移除值
    const next = stack.pop();
    if (Array.isArray(next)) {
      // 使用 push 送回内层数组中的元素,不会改动原始输入
      stack.push(...next);
    } else {
      res.push(next);
    }
  }
  // 反转恢复原数组的顺序
  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;
}
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9]

reduce + concat + isArray + recursivity

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

//to enable deep level flatten use recursion with reduce and concat
function flatDeep(arr) {
   return arr.reduce((acc, val) => acc.concat(Array.isArray(val) ? flatDeep(val) : val), []);
};

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

Use a stack

//non recursive flatten deep using a stack
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();
}

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

Use Generator function

function* flatten(array) {
    for (const item of array) {
        if (Array.isArray(item)) {
            yield* flatten(item);
        } else {
            yield item;
        }
    }
}

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

Please do not add polyfills on this article. For reference, please check: https://discourse.mozilla.org/t/mdn-rfc-001-mdn-wiki-pages-shouldnt-be-a-distributor-of-polyfills/24500

Specifications

Specification Status Comment
ECMAScript 2019 Finished Initial definition

浏览器兼容性

Update compatibility data on GitHub
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
flat Chrome Full support 69 Edge No support No Firefox Full support 62 IE No support No Opera Full support 56 Safari Full support 12 WebView Android Full support 69 Chrome Android Full support 69 Firefox Android Full support 62 Opera Android Full support 48 Safari iOS Full support 12 Samsung Internet Android Full support 10.0 nodejs Full support 11.0.0

Legend

Full support  
Full support
No support  
No support

参见