JS学习笔记

for循环练习

//1
for (let i = 1; i <= 10; i++) {
    if (i % 2 === 0){
    console.log(i)
    }
}
//2
let num = [3, 6, 9, 12]

for (let s = 0; s < num.length; s++) {
    console.log(num[s]*2)
}
//3
let num = [10, 20, 30]
let sum = 0

for (let s = 0; s < num.length; s++) {
    sum = sum + num[s]
}

console.log(sum)

//4
let arr = [3, 7, 2, 9, 4]
let count = 0

for (let s = 0; s < arr.length; s++) {
    if (arr[s] >5 ) {
        count++
    }
    console.log(count)

filter 与 Map

let products =[

    { title: "Book", stock: 10},
    { title: "Pen", stock: 0}, 
    { title: "Bag", stock: 3}    
]

let result = products
  .filter(p => p.stock > 0)
  .map(p => p.title)

console.log(result)



let logs = [
    { level: "error", msg: "fail"},
    { level: "info", msg: "ok"},
    { level: "error", msg: "fail"},
    { level: "error", msg: "fail"},
    { level: "info", msg: "ok"}
]

let infos = logs.filter(l => l.level === "info")
console.log(infos)

find some every

let users = [
    { name: "Tom", age: 15 },
    { name: "Jack", age: 18 },
    { name: "Tom", age: 22 }
]
let adult = users.find(i => i.age >= 18)
console.log(adult)

let products = [
    { title: "Book", stock: 10 },
    { title: "Pen", stock: 0 },
    { title: "Bag", stock: 5 }
]

let output = products.some(i => i.stock === 0)
console.log(output)

let nums = [12, 18, 20, 30]

let even = nums.every(i => i % 2 === 0)

reduce

let orders = [
    { id: 1, paid: true },
    { id: 2, paid: false },
    { id: 3, paid: true },
    { id: 4, paid: false }
]
let add = orders.reduce((a,b) => {
    if (b.paid) {
        a.paid++
    } else {
        a.unpaid++
    }
    return a
},{
    paid: 0,
    unpaid: 0
})
console.log(add)


let users = [
    { name: "Tom", age: 15},
    { name: "Jack", age: 22},
    { name: "Lucy", age: 18},
    { name: "Ben", age: 12}
]
let obj = users.reduce((a, b) => {
    if (b.age >= 18) {
        a.adult.push(b.name)
    }else {
        a.minor.push(b.name)
    }
    return a
}, {
    adult: [],
    minor: []
})
console.log(obj)

sort

let students = [
  { name:'小明', score: 85 },
  { name:'小红', score: 92 },
  { name:'小刚', score: 78 } 
];

let paixu = students.sort((a, b) => b.score - a.score)
                    .map(({name}) => name);

console.log(paixu);

闭包

function createPowerSource() {
  // 这是被保护的能量，外部看不见
  let energy = 100; 

  // 返回一个对象
  return {
    use: function() {
      energy = energy - 10; // 直接操作函数作用域里的 energy
      console.log(`消耗10点，剩余：${energy}`);
    },
    
    charge: function() {
      energy = energy + 20;
      console.log('增加: ${energy}$')
    }
  };
}