函數式編程

函數式編程

filter的使用

var animals = [
 {name:'Fluffykins',species:'rabbit'},
 {name:'Caro',species:'dog'},
 {name:'Hamilton',species:'dog'},
 {name:'Harold',species:'fish'},
 {name:'Ursula',species:'cat'},
 {name:'Jimmy',species:'fish'}
]

//第一種做法
var dogs = []
for (var i = 0; i < animals.length; i++){
    if(animals[i].species === 'dog'){
        dog.push(animals[i])
    }
}

//第二種做法
var dogs = animals.filter(function(animal){
    return animal.species === 'dog'
})

//優化
let isDog = unction(animal){
    return animal.species === 'dog'
}
var dogs = animals.filter(isDog)

map的使用

var animals = [
 {name:'Fluffykins',species:'rabbit'},
 {name:'Caro',species:'dog'},
 {name:'Hamilton',species:'dog'},
 {name:'Harold',species:'fish'},
 {name:'Ursula',species:'cat'},
 {name:'Jimmy',species:'fish'}
]

//第一種做法
var names = []
for ( var i = 0 ; i < animals.length; i++){
    names.push(animals[i].name)
}

//第二種做法
var names = animals.map(function(animal){
    return animal.name
})

//優化
var names = animals.map((animal) => animal.name)

reduce

var orders = [
 {amount:250},
 {amount:400},
 {amount:100},
 {amount:325}
]

//第一種做法
var totalAmount = 0
for ( var i = 0; i < orders.length; i++){
    totalAmount = orders[i].amount
}

//第二種做法
var totalAmount = orders.reduce(function(sum,order){
    return sum + order.amount
},0)

//優化

var totalAmount = orders.reduce((sum,order) => sum + order.amount
},0)

curry

let dragon = (name,size,element) => 
`${name} is a ${size} dragon that breathes ${element} !`

console.log(dragon('fluffykins','tiny','lightling'))

//curry
let dragon = 
  name => 
    size =>
       element => 
            `${name} is a ${size} dragon that breathes ${element} !`
            
console.log(dragon('fluffykins')('tiny')('lightling'))


//還可以
let dragon = 
  name => 
    size =>
       element => 
            `${name} is a ${size} dragon that breathes ${element} !`
            
let fluffykinDragon =  dragon('fluffykins')

console.log(fluffykinDragon('tiny')('lightling'))

//甚至是這樣
let dragon = 
  name => 
    size =>
       element => 
            `${name} is a ${size} dragon that breathes ${element} !`
            
let fluffykinDragon =  dragon('fluffykins')
let tinyDragon = fluffykinDragon('tiny')
console.log(tinyDragon('lightling'))

//使用lodash
import _ from 'lodash'

let dragon =(name,size,element) => (
     `${name} is a ${size} dragon that breathes ${element} !`
)

dragon = _.curry(dragon)

let fluffykinDragon =  dragon('fluffykins')
let tinyDragon = fluffykinDragon('tiny')

console.log(tinyDragon('lightling'))

let dragons = [
 {name:'fluffykins',element:'lightning'},
 {name:'nomi',element:'lightning'},
 {name:'karo',element:'fire'},
 {name:'doomer',element:'timewrap'},
]

let hasElement = (element,obj) => obj.element === element

let lightingDragons = dragons.filter(x => hasElement('lightning',x)

console.log(lightingDragons)

//使用loadash_curry改造
import _ from 'lodash'

let hasElement = _.curry(element,obj) => obj.element === element

let lightingDragons = dragons.filter(hasElement('lightning'))

console.log(lightingDragons)

遞歸

let categories = [
    {
        id:'animals',
        'parent':null
    },
    {
        id:'mammals',
        'parent':'animals'
    },
    {
        id:'cats',
        'parent':'mammals'
    },
    {
        id:'dogs',
        'parent':'mammals'
    },
    {
        id:'chihuahua',
        'parent':'dogs'
    },
    {
        id:'labrador',
        'parent':'dogs'
    },
    {
        id:'persian',
        'parent':'cats'
    },
    {
        id:'siamese',
        'parent':'cats'
    }
]

let makeTree = (categories,parent) => {
    let node = {
        
    }
    categories.filter(c => c.parent === parent)
    .forEach(c =>
    node[c.id] = makeTree(categories,c.id))
    return node
}


console.log(
    JSON.stringify(
        makeTree(categories,null)
        ,null
        ,2
    )
)