const head = ([x]) => x

const arr = [1, 2, 3]
head(arr) // 1

const tail = ([, ...xs]) => xs

const tail = ([x, ...xs]) => xs

const arr = [1, 2, 3]
tail(arr) // [2, 3]

const def = x => typeof x !== "undefined"

const name = "tom"
def(name) // true
def(age) // false

const copy = arr => [...arr]

let arr = [1, 2, 3]
let copied = copy(arr)
copied.push(4)
arr // [1, 2, 3]
copied // [1, 2, 3, 4]

const reverse = ([x, ...xs]) => def(x) ? [...reverse(xs), x] : []

const arr = [1, 2, 3]
reverse(arr) // [3, 2, 1]

const first = ([x, ...xs], n = 1) => def(x) && n ? [x, ...first(xs, n - 1)] : []

const arr = [1, 2, 3]
first(arr, 2) // [1, 2]

const last = (xs, n = 1) => reverse(first(reverse(xs), n))

const arr = [1, 2, 3]
last(arr, 2) // [2, 3]

最新评论

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