2019 年编写现代 JavaScript 代码的5个小技巧

// condition
function test(fruit) {
 if (fruit == "apple" || fruit == "strawberry") {
  console.log("red");
 }
}

function test(fruit) {
 const redFruits = ["apple", "strawberry", "cherry", "cranberries"];

 if (redFruits.includes(fruit)) {
  console.log("red");
 }
}

const arr = [3, 5, 2, 2, 5, 5];
const unique = [...new Set(arr)];
// [3,5,2]

function unique(array) {
 return Array.from(new Set(array));
}

unique([1, 1, 2, 3]); // [1, 2, 3]

let str = [...new Set("ababbc")].join("");
console.log(str);
// 'abc'

let a = new Set([1, 2, 3]);
let b = new Set([4, 3, 2]);

// 并集
let union = new Set([...a, ...b]);
// Set {1, 2, 3, 4}

// 交集
let intersect = new Set([...a].filter(x => b.has(x)));
// set {2, 3}

// 差集
let difference = new Set([...a].filter(x => !b.has(x)));
// Set {1}

const resultMap = new Map()
 .set(-1, {text:'小于',color:'yellow')
 .set(0, {text:'等于',color:'black')
 .set(1, {text:'大于',color:'green')
 .set(null,{text:'没有物品',color:'red'})

let state = resultMap.get(null)
// {text:'没有物品',color:'red'}

const map = new Map([["F", "no"], ["T", "yes"]]);

for (let key of map.keys) {
 console.log(key);
}
// "F"
// "T"

for (let value of map.value()) {
 console.log(value);
}
// "no"
// "yes"

let foo = [
 {
  name: "Stark",
  age: 21
 },
 {
  name: "Jarvis",
  age: 20
 },
 {
  name: "Pepper",
  age: 16
 }
];

//我们希望得到结构稍微不同，age大于16的对象：
let result = [
 {
  person: {
   name: "Stark",
   age: 21
  },
  friends: []
 },
 {
  person: {
   name: "Jarvis",
   age: 20
  },
  friends: []
 }
];

let result = [];

//有时甚至是普通的for循环
foo.forEach(function(person){
  if(person.age > 16){
    let newItem = {
      person: person,
      friends: [];
    };
    result.push(newItem);
  }
})

let result = foo
 .filter(person => person.age > 16)
 .map(person => ({
  person: person,
  friends: []
 }));

let foo = [1, 2, 3, 4, 5];

//不优雅
function sum(arr) {
 let x = 0;
 for (let i = 0; i < arr.length; i++) {
  x += arr[i];
 }
 return x;
}
sum(foo); // => 15

//优雅
foo.reduce((a, b) => a + b); // => 15

const compose = function(f, g) {
 return function(x) {
  return f(g(x));
 };
};

const compose = (f, g) => x => f(g(x));

const head = arr => arr[0];
const reverse = arr => [].concat(arr).reverse();

const last = compose(head, reverse);
last(["jumpkick", "roundhouse", "uppercut"]);
// "uppercut"

function compose(...funcs){
 if (funcs.length === 0){
   return arg => arg
 }

 if (funcs.length === 1 ){
   return funcs[0]
 }

 return funcs.reduce((a,b)=>(...args) => a(b(...args)))
}

const head = arr => arr[0];
const reverse = arr => [].concat(arr).reverse();
const toUpperCase = str => str.toUpperCase();

const last = compose(head, reverse);

const lastAndUpper = compose(toUpperCase, head, reverse,);

console.log(last(["jumpkick", "roundhouse", "uppercut"]));
// "uppercut"
console.log(lastAndUpper(["jumpkick", "roundhouse", "uppercut"]))
// "UPPERCUT"