# 如何实现一个 flatMap 函数 (头条)
Issue
欢迎在 Gtihub Issue 中回答此问题: Issue 229 (opens new window)
Author
没说不让用 flat；所以有个取巧的办法 const flatMap = arr => arr.flat().map(_ => _);
Author
function flatMap(arr){
let list = []
arr.forEach(item=>{
if(Array.isArray(item)){
const l = flatMap(item)
list.push(...l)
}else{
list.push(item)
}
})
return list
}
Author
Array.prototype.flatMap 已经是 EcmaScript 的标准，看一个例子，它的输出是多少？
[1, 2, [3], 4].flatMap((x) => x + 1);
//=> [2, 3, '31', 5]
很可惜，不是 [2, 3, 4, 5]，原因在于 flatMap 实际上是先 map 再 flat，实现如下
Array.prototype.flatMap = function (mapper) {
return this.map(mapper).flat();
};
而 flat 可以如下实现
const flat = (list) => list.reduce((a, b) => a.concat(b), []);
Author
Array.prototype.FlatMap = function (callback, thisArgs) {
return this.reduce((acc, value) => {
return (acc = acc.concat(callback.call(thisArgs, value)));
});
};
Author
https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array/flat
Author
// 先map再flat
let myFlatMap = function (fn) {
let target = this;
return target.map((i) => fn(i)).flat();
};
Array.prototype.myFlatMap = myFlatMap;
let arr = ["it's Sunny in", "", "California"];
let arr1 = arr.map((x) => x.split(" "));
let arr2 = arr.flatMap((x) => x.split(" "));
let arr3 = arr.myFlatMap((x) => x.split(" "));
console.log(arr1); // [["it's","Sunny","in"],[""],["California"]]
console.log(arr2); // ["it's","Sunny","in", "", "California"]
console.log(arr3); // ["it's","Sunny","in", "", "California"]