ecmascript_ECMAScript 2019（ES2019）/ ES10中的新增功能

左手的ㄟ右手 2022-11-27 04:26 149阅读 0赞

ecmascript

**ECMAScript** (ES for short) is a scripting language specification standardized by ECMA International in ECMA-262 and ISO/IEC 16262. It was created to standardize the JavaScript language, so as to foster multiple independent standard implementations from browser vendors. It evolves every year with new features.

**ECMAScript** (简称ES)是ECMA International在ECMA-262和ISO / IEC 16262中标准化的脚本语言规范。创建该规范是为了对JavaScript语言进行标准化，从而促进浏览器供应商的多种独立标准实现。 它每年都有新功能发展。

The 2019 edition of the ECMAScript specification saw the addition of many new features, and here I’ll cover some of these new features. I personally love how javaScript keeps evolving and improving on a regular basis.

ECMAScript规范的2019年版增加了许多新功能，在这里我将介绍其中一些新功能。 我个人喜欢javaScript如何保持不断发展和改进。

## Array.flat() **(**Array.flat()**)** ##

`Array.flat()` returns a new array with any sub-array(s) flattened. A call to `Array.flat()` without any arguments will only flatten one-level deep. An optional depth argument can be provided or it can just be called consecutively.

`Array.flat()`返回一个新数组，该数组的所有子数组都被展平。 不带任何参数的`Array.flat()`调用`Array.flat()`将其`Array.flat()` 。 可以提供可选的depth参数，也可以仅连续调用它。

### 例子： **(**Examples:**)** ###

let arr = [1, 2, 3, [4, 5, 6, [7, 8, 9, [10, 11, 12]]]];
    
    arr.flat(); // [1, 2, 3, 4, 5, 6, Array(4)];
    
    arr.flat().flat(); // [1, 2, 3, 4, 5, 6, 7, 8, 9, Array(3)];
    
    arr.flat(3); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
    
    // Or, if you're not sure about the depth of the array:
    arr.flat(Infinity); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

## Array.flatMap() **(**Array.flatMap()**)** ##

The `flatMap()` method is identical to the ES6 map method, but also flattens at the same time. The `flatMap()` method first maps each element using a mapping function, then flattens the result into a new array. `flatMap()` is often quite useful, as merging both into one method is slightly more efficient.

`flatMap()`方法与ES6地图方法相同，但同时也可以展平。 `flatMap()`方法首先使用映射函数映射每个元素，然后将结果展平为新数组。 `flatMap()`通常非常有用，因为将两者合并为一种方法效率更高。

### 例子： **(**Examples:**)** ###

let arr = [1, 2, 3, 4, 5];
    
    arr.map(x => [x, x * 2]);
    // [Array(2), Array(2), Array(2)]
    // 0: (2)[1, 2]
    // 1: (2)[2, 4]
    // 2: (2)[3, 6]
    
    arr.flatMap(v => [v, v * 2]);
    // [1, 2, 2, 4, 3, 6, 4, 8, 5, 10]

## String.trimStart()和String.trimEnd() **(**String.trimStart() & String.trimEnd()**)** ##

`String.trimStart()` can be used to trim white space from the start of a string.

`String.trimStart()`可用于从字符串开头修剪空白。

### 例子： **(**Examples:**)** ###

let  greeting =  "    Hello everyone";
    
    console.log(greeting.trimStart());
    // "Hello everyone"

let greeting = "Hello world    ";
    
    console.log(greeting.trimEnd());
    // "Hello world"

## 可选的捕捉绑定 **(**Optional Catch Binding**)** ##

Optional catch binding allows developers to use try/catch without the error parameter inside the catch block.

可选的catch绑定使开发人员可以在try代码块中使用try / catch而不使用error参数。

### 例子： **(**Examples:**)** ###

Before ES2019 we use:

在ES2019之前，我们使用：

try {
      // some code
    }
    catch (err) {
      // error handling code
    }

Now we can use try/catch like this with ES2019:

现在我们可以在ES2019中使用try / catch这样的方法：

try  {
      // some code
    }
    catch {
      // error handling code
    }

## Object.fromEntries() **(**Object.fromEntries()**)** ##

It creates an object or transforms key-value pairs into an object. It only accepts [iterables][] e.g: `Object.fromEntries(someIterable)`.

它创建一个对象或将键值对转换为一个对象。 它仅接受[可迭代对象，][iterables]例如： `Object.fromEntries(someIterable)` 。

### 例子： **(**Examples:**)** ###

let entries = new Map([["name", "john"], ["age", 22]]);
    
    console.log(Object.fromEntries(entries));
    // { name: 'john', age: 22 }

## 符号说明 **(**Symbol.description**)** ##

The read-only description property is a string returning the optional description of Symbol objects.

只读description属性是一个字符串，返回Symbol对象的可选描述。

### 例子： **(**Examples:**)** ###

let mySymbol = `My Symbol`;
    
    let symObj = Symbol(mySymbol);
    
    console.log(symObj) // Symbol(mySymbol);
    
    console.log(String(symObj) === `Symbol(${mySymbol})`); // true
    
    console.log(symObj.description); // "My Symbol"

> 翻译自: [https://www.digitalocean.com/community/tutorials/js-es2019][https_www.digitalocean.com_community_tutorials_js-es2019]

ecmascript

[iterables]: https://www.digitalocean.com/community/tutorials/js-iterables
[https_www.digitalocean.com_community_tutorials_js-es2019]: https://www.digitalocean.com/community/tutorials/js-es2019