重学JS-3-一图掌握JS容器

array.forEach(function(item, index, array) {
  console.log(item, index)
})

let newLength = array.push('Orange')

let last = array.pop()

let first = array.shift()

let newLength = array.unshift('Strawberry')

let pos = fruits.indexOf('Banana')
//lastIndexOf ：最后一个的索引
let pos = fruits.lastIndexOf('Banana')

let removedItem = array.splice(pos, 1)
//从一个索引位置删除多个元素
let removedItem = array.splice(pos, n)
//复制一个数组
let shallowCopy = fruits.slice() //begin和end的默认值是0和结尾

const array3 = array1.concat(array2);

const found = array1.find(element => element > 10);
//findIndex返回索引
const isLargeNumberIndex = array1.findIndex(element => element > 13);

const arr1 = [0, 1, 2, [3, 4]];

console.log(arr1.flat());
// expected output: [0, 1, 2, 3, 4]

const arr2 = [0, 1, 2, [[[3, 4]]]];

console.log(arr2.flat(2));
// expected output: [0, 1, 2, [3, 4]]

let arr1 = ["it's Sunny in", "", "California"];

arr1.map(x => x.split(" "));
// [["it's","Sunny","in"],[""],["California"]]

arr1.flatMap(x => x.split(" "));
// ["it's","Sunny","in", "", "California"]
//相当于map和flat(1)一起用，但效率高一些
arr1.map(x => x.split(" ")).flat(1);
// ["it's","Sunny","in", "", "California"]

array.includes('cat')

const elements = ['Fire', 'Air', 'Water'];

console.log(elements.join('-'));
// expected output: "Fire-Air-Water"

const array1 = [1, 2, 3, 4];
const reducer = (previousValue, currentValue) => previousValue + currentValue;

// 1 + 2 + 3 + 4
console.log(array1.reduce(reducer));
// expected output: 10

// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(reducer, 5));
// expected output: 15

const reversed = array1.reverse();

array.some(element => element % 2 === 0)

var m = new Map([['Michael', 95], ['Bob', 75], ['Tracy', 85]]);
m.get('Michael'); // 95

var m = new Map(); // 空Map
m.set('Adam', 67); // 添加新的key-value
m.set('Bob', 59);
m.has('Adam'); // 是否存在key 'Adam': true
m.get('Adam'); // 67
m.delete('Adam'); // 删除key 'Adam'
m.get('Adam'); // undefined

var s1 = new Set(); // 空Set
var s2 = new Set([1, 2, 3]); // 含1, 2, 3
s.add(4);
s; // Set {1, 2, 3, 4}
s.delete(3);
s; // Set {1, 2, 4}
s.has(1); //true
s.clear(); //清空set

let john = { name: "John" };

let weakMap = new WeakMap();
weakMap.set(john, "...");

john = null; // 覆盖引用

// john 被从内存中删除了！

// 📁 cache.js
let cache = new WeakMap();

// 计算并记结果
function process(obj) {
  if (!cache.has(obj)) {
    let result = /* calculate the result for */ obj;

    cache.set(obj, result);
  }

  return cache.get(obj);
}

// 📁 main.js
let obj = {/* some object */};

let result1 = process(obj);
let result2 = process(obj);

// ……稍后，我们不再需要这个对象时：
obj = null;

// 无法获取 cache.size，因为它是一个 WeakMap，
// 要么是 0，或即将变为 0
// 当 obj 被垃圾回收，缓存的数据也会被清除