147 lines
4.1 KiB
Markdown
147 lines
4.1 KiB
Markdown
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---
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title: Spread syntax
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localeTitle: 传播语法
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---
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## 传播语法
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Spread语法允许在可能需要零个或多个参数(用于函数调用)或元素(用于数组文字)的位置扩展可迭代的数组表达式或字符串,或者在预期为零的位置展开对象表达式。
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### 句法
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对于函数调用:
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```
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myFunction(...iterableObj);
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```
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对于数组文字或字符串:
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```
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[...iterableObj, '4', 'five', 6];
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```
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### 例子
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#### 传播函数调用
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#### 替换申请
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在要使用数组元素作为函数参数的情况下,通常使用`Function.prototype.apply` 。
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```
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function myFunction(x, y, z) { }
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var args = [0, 1, 2];
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myFunction.apply(null, args);
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```
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使用扩展语法,上面可以写成:
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```
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function myFunction(x, y, z) { }
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var args = [0, 1, 2];
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myFunction(...args);
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```
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参数列表中的任何参数都可以使用扩展语法,并且可以多次使用。
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```
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function myFunction(v, w, x, y, z) { }
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var args = [0, 1];
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myFunction(-1, ...args, 2, ...[3]);
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```
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### 申请新的
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当使用`new`调用构造函数时,不可能**直接**使用数组并`apply` ( `apply`执行`[[Call]]`而不是`[[Construct]]` )。但是,由于扩展语法,可以很容易地使用数组:
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```
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var dateFields = [1970, 0, 1]; // 1 Jan 1970
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var d = new Date(...dateFields);
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```
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要使用带有参数数组的new而不使用扩展语法,您必须通过部分应用程序**间接**执行此操作:
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```
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function applyAndNew(constructor, args) {
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function partial () {
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return constructor.apply(this, args);
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};
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if (typeof constructor.prototype === "object") {
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partial.prototype = Object.create(constructor.prototype);
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}
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return partial;
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}
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function myConstructor () {
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console.log("arguments.length: " + arguments.length);
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console.log(arguments);
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this.prop1="val1";
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this.prop2="val2";
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};
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var myArguments = ["hi", "how", "are", "you", "mr", null];
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var myConstructorWithArguments = applyAndNew(myConstructor, myArguments);
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console.log(new myConstructorWithArguments);
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// (internal log of myConstructor): arguments.length: 6
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// (internal log of myConstructor): ["hi", "how", "are", "you", "mr", null]
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// (log of "new myConstructorWithArguments"): {prop1: "val1", prop2: "val2"}
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```
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### 传播数组文字
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#### 一个更强大的数组文字
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如果没有扩展语法,要使用现有数组作为其中一部分创建新数组,则数组文字语法不再足够,必须使用push,splice,concat等组合使用命令式代码。使用扩展语法变得更加简洁:
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```
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var parts = ['shoulders', 'knees'];
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var lyrics = ['head', ...parts, 'and', 'toes'];
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// ["head", "shoulders", "knees", "and", "toes"]
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```
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就像扩展参数列表一样, `...`可以在数组文字中的任何地方使用,并且可以多次使用。
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### 复制一个数组
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```
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var arr = [1, 2, 3];
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var arr2 = [...arr]; // like arr.slice()
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arr2.push(4);
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// arr2 becomes [1, 2, 3, 4]
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// arr remains unaffected
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```
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> **注意** :在复制数组时,Spread语法有效地深入一级。因此,它可能不适合复制多维数组,如下例所示(与Object.assign()和spread语法相同)。
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```
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var a = [[1], [2], [3]];
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var b = [...a];
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b.shift().shift(); // 1
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// Now array a is affected as well: [[], [2], [3]]
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```
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### 连接数组的更好方法
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`Array.concat`通常用于将数组连接到现有数组的末尾。没有扩展语法,这可以完成:
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```
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var arr1 = [0, 1, 2];
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var arr2 = [3, 4, 5];
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// Append all items from arr2 onto arr1
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arr1 = arr1.concat(arr2);
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```
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使用扩展语法,这将成为:
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```
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var arr1 = [0, 1, 2];
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var arr2 = [3, 4, 5];
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arr1 = [...arr1, ...arr2];
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```
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`Array.unshift`通常用于在现有数组的开头插入值数组。没有扩展语法,这可以完成:
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```
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var arr1 = [0, 1, 2];
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var arr2 = [3, 4, 5];
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// Prepend all items from arr2 onto arr1
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Array.prototype.unshift.apply(arr1, arr2) // arr1 is now [3, 4, 5, 0, 1, 2]
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```
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使用扩展语法,这将成为:
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```
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var arr1 = [0, 1, 2];
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var arr2 = [3, 4, 5];
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arr1 = [...arr2, ...arr1]; // arr1 is now [3, 4, 5, 0, 1, 2]
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```
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