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261 lines
18 KiB
HTML
<article id="wikiArticle">
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<div></div>
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<p><code><strong>Object.assign()</strong></code> 方法用于将所有可枚举属性的值从一个或多个源对象复制到目标对象。它将返回目标对象。</p>
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<div><iframe class="interactive interactive-js" frameborder="0" height="250" src="https://interactive-examples.mdn.mozilla.net/pages/js/object-assign.html" width="100%"></iframe></div>
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<p class="hidden">The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone <a class="external" href="https://github.com/mdn/interactive-examples" rel="noopener">https://github.com/mdn/interactive-examples</a> and send us a pull request.</p>
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<h2 id="Syntax" name="Syntax">语法</h2>
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<pre><code class="language-javascript"><code>Object.assign(<var>target</var>, ...<var>sources</var>)</code></code></pre>
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<h3 id="Parameters" name="Parameters">参数</h3>
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<dl>
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<dt><code>target</code></dt>
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<dd>目标对象。</dd>
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<dt><code>sources</code></dt>
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<dd>源对象。</dd>
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</dl>
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<h3 id="Return_value" name="Return_value">返回值</h3>
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<p>目标对象。</p>
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<h2 id="描述">描述</h2>
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<p>如果目标对象中的属性具有相同的键，则属性将被源对象中的属性覆盖。后面的源对象的属性将类似地覆盖前面的源对象的属性。</p>
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<p><code>Object.assign</code> 方法只会拷贝源对象自身的并且可枚举的属性到目标对象。该方法使用源对象的<code>[[Get]]</code>和目标对象的<code>[[Set]]</code>，所以它会调用相关 getter 和 setter。因此，它分配属性，而不仅仅是复制或定义新的属性。如果合并源包含getter，这可能使其不适合将新属性合并到原型中。为了将属性定义（包括其可枚举性）复制到原型，应使用<a href="Reference/Global_Objects/Object/getOwnPropertyDescriptor" title="Object.getOwnPropertyDescriptor() 方法返回指定对象上一个自有属性对应的属性描述符。（自有属性指的是直接赋予该对象的属性，不需要从原型链上进行查找的属性）"><code>Object.getOwnPropertyDescriptor()</code></a>和<a href="Reference/Global_Objects/Object/defineProperty" title="Object.defineProperty() 方法会直接在一个对象上定义一个新属性，或者修改一个对象的现有属性， 并返回这个对象。"><code>Object.defineProperty()</code></a> 。</p>
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<p><a href="Reference/String" title="此页面仍未被本地化, 期待您的翻译!"><code>String</code></a>类型和 <a href="Reference/Global_Objects/Symbol" title='Symbol()函数会返回symbol类型的值，该类型具有静态属性和静态方法。它的静态属性会暴露几个内建的成员对象；它的静态方法会暴露全局的symbol注册，且类似于内建对象类，但作为构造函数来说它并不完整，因为它不支持语法："new Symbol()"。'><code>Symbol</code></a> 类型的属性都会被拷贝。</p>
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<p>在出现错误的情况下，例如，如果属性不可写，会引发<a href="Reference/Global_Objects/TypeError" title="TypeError（类型错误） 对象用来表示值的类型非预期类型时发生的错误。"><code>TypeError</code></a>，如果在引发错误之前添加了任何属性，则可以更改<code>target</code>对象。</p>
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<p>注意，<code>Object.assign</code> 不会跳过那些值为 <a href="Reference/Global_Objects/null" title="值 null 特指对象的值未设置。它是 JavaScript 基本类型 之一。"><code>null</code></a> 或 <a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说，它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a> 的源对象。</p>
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<h2 id="Examples" name="Examples">示例</h2>
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<h3 id="Example:_Cloning_an_object" name="Example:_Cloning_an_object">复制一个对象</h3>
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<pre><code class="language-javascript">const obj = { a: 1 };
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const copy = Object.assign({}, obj);
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console.log(copy); // { a: 1 }
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</code></pre>
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<h3 id="Deep_Clone" name="Deep_Clone">深拷贝问题</h3>
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<p>针对深拷贝，需要使用其他办法，因为 <code>Object.assign()</code>拷贝的是属性值。假如源对象的属性值是一个对象的引用，那么它也只指向那个引用。</p>
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<pre><code class="language-javascript">let obj1 = { a: 0 , b: { c: 0}};
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let obj2 = Object.assign({}, obj1);
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console.log(JSON.stringify(obj2)); // { a: 0, b: { c: 0}}
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obj1.a = 1;
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console.log(JSON.stringify(obj1)); // { a: 1, b: { c: 0}}
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console.log(JSON.stringify(obj2)); // { a: 0, b: { c: 0}}
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obj2.a = 2;
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console.log(JSON.stringify(obj1)); // { a: 1, b: { c: 0}}
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console.log(JSON.stringify(obj2)); // { a: 2, b: { c: 0}}
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obj2.b.c = 3;
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console.log(JSON.stringify(obj1)); // { a: 1, b: { c: 3}}
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console.log(JSON.stringify(obj2)); // { a: 2, b: { c: 3}}
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// Deep Clone
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obj1 = { a: 0 , b: { c: 0}};
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let obj3 = JSON.parse(JSON.stringify(obj1));
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obj1.a = 4;
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obj1.b.c = 4;
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console.log(JSON.stringify(obj3)); // { a: 0, b: { c: 0}}</code></pre>
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<h3 id="Example:_Merging_objects" name="Example:_Merging_objects">合并对象</h3>
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<pre><code class="language-javascript">const o1 = { a: 1 };
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const o2 = { b: 2 };
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const o3 = { c: 3 };
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const obj = Object.assign(o1, o2, o3);
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console.log(obj); // { a: 1, b: 2, c: 3 }
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console.log(o1); // { a: 1, b: 2, c: 3 }, 注意目标对象自身也会改变。
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</code></pre>
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<h3 id="合并具有相同属性的对象">合并具有相同属性的对象</h3>
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<pre><code class="language-javascript">const o1 = { a: 1, b: 1, c: 1 };
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const o2 = { b: 2, c: 2 };
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const o3 = { c: 3 };
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const obj = Object.assign({}, o1, o2, o3);
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console.log(obj); // { a: 1, b: 2, c: 3 }</code></pre>
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<p>属性被后续参数中具有相同属性的其他对象覆盖。</p>
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<h3 id="Example:_Symbol_properties" name="Example:_Symbol_properties">拷贝 symbol 类型的属性</h3>
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<pre><code class="language-javascript">const o1 = { a: 1 };
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const o2 = { [Symbol('foo')]: 2 };
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const obj = Object.assign({}, o1, o2);
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console.log(obj); // { a : 1, [Symbol("foo")]: 2 } (cf. bug 1207182 on Firefox)
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Object.getOwnPropertySymbols(obj); // [Symbol(foo)]</code></pre>
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<h3 id="Example:_Only_own_enumerable_properties" name="Example:_Only_own_enumerable_properties">继承属性和不可枚举属性是不能拷贝的</h3>
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<pre><code class="language-javascript">const obj = Object.create({foo: 1}, { // foo 是个继承属性。
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bar: {
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value: 2 // bar 是个不可枚举属性。
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},
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baz: {
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value: 3,
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enumerable: true // baz 是个自身可枚举属性。
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}
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});
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const copy = Object.assign({}, obj);
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console.log(copy); // { baz: 3 }
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</code></pre>
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<h3 id="Example:_Primitives" name="Example:_Primitives">原始类型会被包装为对象</h3>
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<pre><code class="language-javascript">const v1 = "abc";
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const v2 = true;
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const v3 = 10;
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const v4 = Symbol("foo")
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const obj = Object.assign({}, v1, null, v2, undefined, v3, v4);
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// 原始类型会被包装，null 和 undefined 会被忽略。
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// 注意，只有字符串的包装对象才可能有自身可枚举属性。
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console.log(obj); // { "0": "a", "1": "b", "2": "c" }</code></pre>
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<h3 id="Example:_Exceptions" name="Example:_Exceptions">异常会打断后续拷贝任务</h3>
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<pre><code class="language-javascript">const target = Object.defineProperty({}, "foo", {
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value: 1,
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writable: false
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}); // target 的 foo 属性是个只读属性。
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Object.assign(target, {bar: 2}, {foo2: 3, foo: 3, foo3: 3}, {baz: 4});
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// TypeError: "foo" is read-only
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// 注意这个异常是在拷贝第二个源对象的第二个属性时发生的。
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console.log(target.bar); // 2，说明第一个源对象拷贝成功了。
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console.log(target.foo2); // 3，说明第二个源对象的第一个属性也拷贝成功了。
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console.log(target.foo); // 1，只读属性不能被覆盖，所以第二个源对象的第二个属性拷贝失败了。
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console.log(target.foo3); // undefined，异常之后 assign 方法就退出了，第三个属性是不会被拷贝到的。
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console.log(target.baz); // undefined，第三个源对象更是不会被拷贝到的。
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</code></pre>
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<h3 id="Example:_Copy_accessors" name="Example:_Copy_accessors">拷贝访问器</h3>
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<pre><code class="language-javascript">const obj = {
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foo: 1,
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get bar() {
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return 2;
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}
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};
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let copy = Object.assign({}, obj);
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console.log(copy); // { foo: 1, bar: 2 } copy.bar的值来自obj.bar的getter函数的返回值
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// 下面这个函数会拷贝所有自有属性的属性描述符
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function completeAssign(target, ...sources) {
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sources.forEach(source => {
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let descriptors = Object.keys(source).reduce((descriptors, key) => {
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descriptors[key] = Object.getOwnPropertyDescriptor(source, key);
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return descriptors;
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}, {});
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// Object.assign 默认也会拷贝可枚举的Symbols
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Object.getOwnPropertySymbols(source).forEach(sym => {
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let descriptor = Object.getOwnPropertyDescriptor(source, sym);
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if (descriptor.enumerable) {
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descriptors[sym] = descriptor;
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}
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});
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Object.defineProperties(target, descriptors);
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});
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return target;
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}
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copy = completeAssign({}, obj);
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console.log(copy);
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// { foo:1, get bar() { return 2 } }
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</code></pre>
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<h2 id="Polyfill" name="Polyfill">Polyfill</h2>
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<p>此<a class="glossaryLink" href="/en-US/docs/Glossary/Polyfill" title="polyfill: A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.">polyfill</a>不支持 symbol 属性，因为ES5 中根本没有 symbol ：</p>
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<pre><code class="language-javascript">if (typeof Object.assign != 'function') {
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// Must be writable: true, enumerable: false, configurable: true
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Object.defineProperty(Object, "assign", {
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value: function assign(target, varArgs) { // .length of function is 2
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'use strict';
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if (target == null) { // TypeError if undefined or null
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throw new TypeError('Cannot convert undefined or null to object');
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}
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let to = Object(target);
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for (var index = 1; index < arguments.length; index++) {
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var nextSource = arguments[index];
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if (nextSource != null) { // Skip over if undefined or null
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for (let nextKey in nextSource) {
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// Avoid bugs when hasOwnProperty is shadowed
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if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
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to[nextKey] = nextSource[nextKey];
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}
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}
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}
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}
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return to;
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},
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writable: true,
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configurable: true
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});
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}</code></pre>
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<h2 id="Specifications" name="Specifications">规范</h2>
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<table class="standard-table">
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<tbody>
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<tr>
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<th scope="col">规范名称</th>
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<th scope="col">规范状态</th>
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<th scope="col">备注</th>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-object.assign" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Object.assign</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td>Initial definition.</td>
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</tr>
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<tr>
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<td><a class="external" href="https://tc39.github.io/ecma262/#sec-object.assign" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Object.assign</small></a></td>
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<td><span class="spec-Draft">Draft</span></td>
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<td> </td>
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</tr>
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</tbody>
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</table>
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<h2 id="Browser_compatibility" name="Browser_compatibility">浏览器兼容</h2>
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<div class="hidden">The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out <a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</div>
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<p></p><div class="bc-data"><a class="bc-github-link external" href="https://github.com/mdn/browser-compat-data" rel="noopener">Update compatibility data on GitHub</a><table class="bc-table bc-table-js"><thead><tr class="bc-platforms"><td></td><th class="bc-platform-desktop" colspan="6"><span>Desktop</span></th><th class="bc-platform-mobile" colspan="7"><span>Mobile</span></th><th class="bc-platform-server" colspan="1"><span>Server</span></th></tr><tr class="bc-browsers"><td></td><th class="bc-browser-chrome"><span class="bc-head-txt-label bc-head-icon-chrome">Chrome</span></th><th class="bc-browser-edge"><span class="bc-head-txt-label bc-head-icon-edge">Edge</span></th><th class="bc-browser-firefox"><span class="bc-head-txt-label bc-head-icon-firefox">Firefox</span></th><th class="bc-browser-ie"><span class="bc-head-txt-label bc-head-icon-ie">Internet Explorer</span></th><th class="bc-browser-opera"><span class="bc-head-txt-label bc-head-icon-opera">Opera</span></th><th class="bc-browser-safari"><span class="bc-head-txt-label bc-head-icon-safari">Safari</span></th><th class="bc-browser-webview_android"><span class="bc-head-txt-label bc-head-icon-webview_android">Android webview</span></th><th class="bc-browser-chrome_android"><span class="bc-head-txt-label bc-head-icon-chrome_android">Chrome for Android</span></th><th class="bc-browser-edge_mobile"><span class="bc-head-txt-label bc-head-icon-edge_mobile">Edge Mobile</span></th><th class="bc-browser-firefox_android"><span class="bc-head-txt-label bc-head-icon-firefox_android">Firefox for Android</span></th><th class="bc-browser-opera_android"><span class="bc-head-txt-label bc-head-icon-opera_android">Opera for Android</span></th><th class="bc-browser-safari_ios"><span class="bc-head-txt-label bc-head-icon-safari_ios">Safari on iOS</span></th><th class="bc-browser-samsunginternet_android"><span class="bc-head-txt-label bc-head-icon-samsunginternet_android">Samsung Internet</span></th><th class="bc-browser-nodejs"><span class="bc-head-txt-label bc-head-icon-nodejs">Node.js</span></th></tr></thead><tbody><tr><th scope="row"><code>assign</code></th><td class="bc-supports-yes bc-browser-chrome"><span class="bc-browser-name">Chrome</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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45</td><td class="bc-supports-yes bc-browser-edge"><span class="bc-browser-name">Edge</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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12</td><td class="bc-supports-yes bc-browser-firefox"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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34</td><td class="bc-supports-no bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-no only-icon" title="No support">
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<span>No support</span>
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</abbr>
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No</td><td class="bc-supports-yes bc-browser-opera"><span class="bc-browser-name">Opera</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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32</td><td class="bc-supports-yes bc-browser-safari"><span class="bc-browser-name">Safari</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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9</td><td class="bc-supports-no bc-browser-webview_android"><span class="bc-browser-name">WebView Android</span><abbr class="bc-level-no only-icon" title="No support">
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<span>No support</span>
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</abbr>
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No</td><td class="bc-supports-yes bc-browser-chrome_android"><span class="bc-browser-name">Chrome Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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45</td><td class="bc-supports-yes bc-browser-edge_mobile"><span class="bc-browser-name">Edge Mobile</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-firefox_android"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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34</td><td class="bc-supports-yes bc-browser-opera_android"><span class="bc-browser-name">Opera Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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32</td><td class="bc-supports-yes bc-browser-safari_ios"><span class="bc-browser-name">Safari iOS</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-samsunginternet_android"><span class="bc-browser-name">Samsung Internet Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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5.0</td><td class="bc-supports-yes bc-browser-nodejs"><span class="bc-browser-name">nodejs</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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4.0.0</td></tr></tbody></table><section class="bc-legend" id="sect1"><h3 class="offscreen" id="Legend">Legend</h3><dl><dt><span class="bc-supports-yes bc-supports">
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<abbr class="bc-level bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr></span></dt><dd>Full support</dd><dt><span class="bc-supports-no bc-supports">
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<abbr class="bc-level bc-level-no only-icon" title="No support">
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<span>No support</span>
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</abbr></span></dt><dd>No support</dd></dl></section></div><p></p>
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<h2 id="See_also" name="See_also">相关链接</h2>
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<ul>
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<li><a href="Reference/Global_Objects/Object/defineProperties" title="Object.defineProperties() 方法直接在一个对象上定义新的属性或修改现有属性，并返回该对象。"><code>Object.defineProperties()</code></a></li>
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<li><a href="Enumerability_and_ownership_of_properties">属性的可枚举性和所有权</a></li>
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</ul>
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</article> |