函数式编程（FP）就是利用纯函数来构建软件的过程，避免共享状态，可变数组以及一些副作用。而 ES6 带来了很多的特性帮助我们更方便地进行 JavaScript 函数式编程。
今天我们就来看看 ES6 的剩余参数语法结合递归实现如何实现函数式编程，用函数式思想实现一些我们常用的方法。
Head
返回数组的第一个元素
const head = ([x]) => x使用
const arr = [1, 2, 3]
head(arr) // 1Tail
返回数组中除第一个元素外的其它元素
const tail = ([, ...xs]) => xs上面写法中省略了第一个参数，所以也等价于
const tail = ([x, ...xs]) => xs使用
const arr = [1, 2, 3]
tail(arr) // [2, 3]Def
如果存在参数则返回 true，否则返回 false
const def = x => typeof x !== "undefined"使用
const name = "tom"
def(name) // true
def(age) // falseCopy
复制数组
const copy = arr => [...arr]使用
let arr = [1, 2, 3]
let copied = copy(arr)
copied.push(4)
arr // [1, 2, 3]
copied // [1, 2, 3, 4]Reverse
反转数组
const reverse = ([x, ...xs]) => def(x) ? [...reverse(xs), x] : []这里我们通过递归依次将整个数组给调换顺序了，使用
const arr = [1, 2, 3]
reverse(arr) // [3, 2, 1]First
返回一个新的数组，包含给定数组前 n 项的元素
const first = ([x, ...xs], n = 1) => def(x) && n ? [x, ...first(xs, n - 1)] : []同样运用了递归的思想，使用
const arr = [1, 2, 3]
first(arr, 2) // [1, 2]计算过程如下
- 首先
first(arr, 2)=first([1, 2, 3], 2) - def(x) = def(1) && n = 2 =>
true - 返回
[x, ...first(xs, n - 1)]=[1, ...first([2, 3], 1)] - 计算
first([2, 3], 1) - def(x) = def(2) && n = 1 =>
true - 返回
[x, ...first(xs, n - 1)]=[2, ...first([3], 0)] - 计算
first([3], 0) - def(x) = def(3) && n = 0 =>
false - 于是
first([3], 0)返回[] - 所以结果为
[1, ...[2, ...[]]]=>[1, 2]
Last
返回一个新的数组，包含给定数组后 n 项的元素
const last = (xs, n = 1) => reverse(first(reverse(xs), n))结合上面的 first 方法和反转，使用
const arr = [1, 2, 3]
last(arr, 2) // [2, 3]本期的主要目的不是为了让大家在实际开发中就这样去上面的方法，实际上数组也提供了相应的便捷方法，之所以讲这些，是为了让大家更加深刻理解函数式编程的纯函数特性。
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