模式匹配 (pattern matching) 是 lisp、rust、scala、erlang 等语言中的常见语法,不过 JavaScript 始终未反对这个个性,尽管 tc39 早有相干的题案 https://github.com/tc39/proposal-pattern-matching,不过目前停顿迟缓,遂研发了一个 Babel 插件用于在 JavaScript 中反对模式匹配的语法
babel-plugin-proposal-pattern-matching
装置
应用 npm:
npm install --save-dev babel-plugin-proposal-pattern-matching
设置
.babelrc
{"plugins": ["babel-plugin-proposal-pattern-matching"]
}
一些演示
简略应用
import match from 'babel-plugin-proposal-pattern-matching/match'
const fib = n=>match(n)((v=1)=>1,
(v=2)=>1,
_=>fib(_-1)+fib(_-2)
)
console.log(fib(10))
// -> 55
间接返回一个函数,配合函数式编程
import match from 'babel-plugin-proposal-pattern-matching/match'
const fib = fnmatch((v = 1) => 1,
(v = 2) => 1,
_ => fib(_ - 1) + fib(_ - 2)
)
console.log(fib(10))
// -> 55
const arr = [1, 2, 3]
console.log(
arr.map(
fnmatch((v = 1) => 'one',
(v = 2) => 'two',
(v = 3) => 'three'
)
)
)
// -> ['one', 'two', 'three']
数据类型判断
import {match , T} from 'babel-plugin-proposal-pattern-matching/match'
const getType = item => match(item)((v = T.number) => 'number',
(v = T.string) => 'string',
(v = T.nullish) => 'nullish',
_ => `${_} undefined type`
)
console.log(getType('a'))
// -> string
console.log(getType(1))
// -> number
console.log(getType(undefined))
// -> nullish
console.log(getType(null))
// -> nullish
实例判断
import {match, instanceOf} from 'babel-plugin-proposal-pattern-matching/match'
const getType = val => match(val)((v=instanceOf(RegExp))=>'RegExp',
(v=instanceOf(Array))=>'Array',
(v=instanceOf(Object))=>'Object',
)
console.log(getType(/111/))
// -> RegExp
console.log(getType([1,2,3]))
// -> Array
console.log(getType({a:1}))
// -> Object
解构,这里反对有限嵌套
import {match} from 'babel-plugin-proposal-pattern-matching/match'
const sum = x => match(x)(([x, ...xs]) => x + sum(xs),
([]) => 0
)
console.log(sum([1, 2, 3]))
// -> 6
for (let i = 1; i <= 15; i++) {
console.log(match({a: i % 3, b: i % 5})(({a = 0, b = 0}) => 'FizzBuzz',
({a = 0, b}) => 'Fizz',
({a, b = 0}) => 'Buzz',
_ => i
)
)
}
// ->
// 1
// 2
// Fizz
// 4
// Buzz
// Fizz
// 7
// 8
// Fizz
// Buzz
// 11
// Fizz
// 13
// 14
// FizzBuzz
非运算
import {match, not, or, T} from 'babel-plugin-proposal-pattern-matching/match'
const toNumber = n => match(n)((v = not(T.boolean)) => v,
(v = true) => 1,
(v = false) => 0
)
console.log([true, false, 0, 1, 2, 3].map(toNumber)
)
// -> [1, 0, 0, 1, 2, 3]
或运算
import {match, or} from 'babel-plugin-proposal-pattern-matching/match'
const fib = n => match(n)((v = or(1, 2)) => 1,
_ => fib(_ - 1) + fib(_ - 2)
)
console.log(fib(10))
// -> 55
与运算
import {match, and, not} from 'babel-plugin-proposal-pattern-matching/match'
const fib = n => match(n)((_ = and(not(1), not(2))) => fib(_ - 1) + fib(_ - 2),
_ => 1
)
console.log(fib(10))
// -> 55