JavaScript中的函数与命令模式，Java也有参考价值


这篇文章的目的并不是说一种范式比另一种范式更好。它只是显示您遇到的常见模式及其同等的函数实现方法。(banq注：其实对使用Java Stream替代if/else也有参考作用)

if / else
命令范式：

const hour = 14
let greeting

if (hour < 18) {
greeting = 'Good day';
} else {
greeting = 'Good evening';
}

函数范式：
if语句可以被函数getGreeting替代，从而重用：
const isDay = hour => hour < 18
const getGreeting = hour => isDay(hour) ? 'Good Day' : 'Good Evening'
const greeting = getGreeting (hour)


if (无else)
命令范式：

let name = 'joel'
if (name != null) {
name = name.toUpperCase()
}


函数范式：
isNotnull和toUpperCase市可重用的函数：

import when from 'mojiscript/logic/when'

const isNotNull = obj => obj != null
const toUpperCase = when (isNotNull) (string => string.toUpperCase ())

const name = 'joel'
const upperName = toUpperCase (name)


数组相加
命令范式：

const values = [1, 2, 3]

let sum = 0
for (const x of values) {
sum = sum + x
}

函数范式：
不将sum变成可变变量：

const values = [1, 2, 3]

const add = (x, y) => x + y
const sum = values.reduce(add)

[b]for/if[/b]

命令范式：
const values = [1, 2, 3, 4, 5]

let evens =
for (const x of values) {
if (x % 2 === 0) {
evens.push(x)
}
}

函数范式：
不要将evens成为可变：

import filter from 'mojiscript/list/filter'

const values = [1, 2, 3, 4, 5]

const isEven = num => num % 2 === 0
const evens = filter (isEven) (values)


提前中断循环
命令范式：

const values = [1, 2, 3]
let sum = 0
for (const x of values) {
if (x > 3) break
sum = sum + x
}

函数范式：
使用reduceWhile，类似reduce，能接受一个predicate提早结束。

import reduceWhile from 'mojiscript/list/reduceWhile'

const add = x => y => x + y
const lte3 = num => num <= 3

const sum = reduceWhile (() => lte3) (add) (0) (values)

if / else if / else
命令范式：

const fn = temp => {
if (temp === 0) return 'water freezes at 0°C'
else if (temp === 100) return 'water boils at 100°C'
else return `nothing special happens at ${temp}°C`
}

fn(0) //=> 'water freezes at 0°C'
fn(50) //=> 'nothing special happens at 50°C'
fn(100) //=> 'water boils at 100°C'

函数范式：

import cond from 'mojiscript/logic/cond'
import $ from 'mojiscript/string/template'

const fn = cond([
[0, 'water freezes at 0°C'],
[100, 'water boils at 100°C'],
[() => true, $`nothing special happens at ${0}°C`]
])

fn(0) //=> 'water freezes at 0°C'
fn(50) //=> 'nothing special happens at 50°C'
fn(100) //=> 'water boils at 100°C'

类
命令范式：

class Cat {
constructor() {
this.sound = 'Meow'
}

talk() {
return this.sound
}
}

const cat = new Cat()
const talk = cat.talk

cat.talk() //=> 'Meow'
talk() //=> Error: Cannot read property 'sound' of undefined


函数范式：

const cat = {
sound: 'Meow'
}

const dog = {
sound: 'Woof'
}

const talk = animal => animal.sound

talk (cat) //=> 'Meow'
talk (dog) //=> 'Woof'

检查空NULL
命令范式：

const toUpper = string => {
if (string != null) {
return string.toUpperCase()
}
}


函数范式：
此示例将参数包装在一个

Maybe
类型中，然后在最后将其解包。在一般FP应用程序中，可在整个应用程序使用Maybe，不需要打包和解包string。所以这比你通常看到的要冗长一些。

import S from 'sanctuary'

const toUpper = S.pipe ([
S.toMaybe,
S.map (string => string.toUpperCase ()),
S.maybeToNullable
])

// If you use `Maybe` throughout your app, this would be your `toUpper` function.
const toUpper = S.map (string => string.toUpperCase ());

good !!