26.ES10 新特性

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1.Object.fromEntries()

Object.fromEntries() 方法可以将 二维数组 和 Map 转换成对象

(1) 二维数组

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// 二维数组
const result = Object.fromEntries([
    ["name", "Tom"],
    ["age", 20],
    ["address", "street"]
]);
console.log(result);
// {name: 'Tom', age: 20, address: 'street'}

(2) Map

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// Map
const m = new Map();
m.set("name", "Tom");
m.set("age", 20);
m.set("address", "street");
const result = Object.fromEntries(m);
console.log(result);
// {name: 'Tom', age: 20, address: 'street'}

(3) ES8 Object.entries()

Object.fromEntries() 与 Object.entries() 互为逆运算

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// Object.entries() ES8 特性
const arr = Object.entries({
    name: "Tom",
    age: 20,
    address: "street"
});
console.log(arr);
// (3) [Array(2), Array(2), Array(2)]
//     0: (2) ['name', 'Tom']
//     1: (2) ['age', 20]
//     2: (2) ['address', 'street']
//     length: 3
//     [[Prototype]]: Array(0)

2.trimStart() 与 trimEnd()

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trimStart(): 去除字符串左侧空格
trimEnd(): 去除字符串右侧空格
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// trim
let str = "   Hello world   123   ";

console.log(str.trim());
// Hello world   123
console.log(str.trimStart());
// Hello world   123   
console.log(str.trimEnd());
//    Hello world   123

3.flat() 与 flatMap()

(1) flat()

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// flat()
// 将多维数组转换为低维数组
const arr1 = [1, 2, [3, 4], 5, [6, 7, 8], 9];
console.log(arr1.flat());    // (9) [1, 2, 3, 4, 5, 6, 7, 8, 9]

const arr2 = [1, 2, [3, 4, [5, 6, 7], 8], 9];
console.log(arr2.flat());   // (7) [1, 2, 3, 4, Array(3), 8, 9]
console.log(arr2.flat(2));  // (9) [1, 2, 3, 4, 5, 6, 7, 8, 9]

(2) flatMap()

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const arr3 = [1, 2, 3, 4];
const result = arr3.map(item => item * 10);
console.log(result);    // (4) [10, 20, 30, 40]
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const arr3 = [1, 2, 3, 4];
const result = arr3.map(function(item) { 
    return [item * 10];
});
console.log(result);
// (4) [Array(1), Array(1), Array(1), Array(1)]
//     0: [10]
//     1: [20]
//     2: [30]
//     3: [40]
//     length: 4
//     [[Prototype]]: Array(0)

使用 flatMap() 对元素进行展开

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const arr4 = [1, 2, 3, 4];
const result = arr4.flatMap(item => [item * 10]);
console.log(result);
// (4) [10, 20, 30, 40]

4.Symbol.prototype.description

Symbol 对象的 description 属性值为 Symbol 对象的字符串的值

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// 创建 Symbol 对象
let s = Symbol("SymbolTestObj");

console.log(s.description);
// SymbolTestObj