uTools-Manuals/docs/javascript/Reference/Global_Objects/Object/constructor.html

<article id="wikiArticle">
<p></p><p></p>
<p>返回创建实例对象的 <a href="Reference/Global_Objects/Object" title="Object 构造函数创建一个对象包装器。"><code>Object</code></a> 构造函数的引用。注意，此属性的值是对函数本身的引用，而不是一个包含函数名称的字符串。对原始类型来说，如<code>1</code>，<code>true</code>和<code>"test"</code>，该值只可读。</p>
<h2 id="Description" name="Description">描述</h2>
<p>所有对象都会从它的原型上继承一个 <code>constructor</code> 属性：</p>
<pre><code  class="language-javascript">var o = {};
o.constructor === Object; // true

var o = new Object;
o.constructor === Object; // true

var a = [];
a.constructor === Array; // true

var a = new Array;
a.constructor === Array // true

var n = new Number(3);
n.constructor === Number; // true</code></pre>
<h2 id="Examples" name="Examples">示例</h2>
<h3 id="Example:_Displaying_the_constructor_of_an_object" name="Example:_Displaying_the_constructor_of_an_object">打印一个对象的构造函数</h3>
<p>以下示例创建一个原型，<code>Tree</code>，以及该类型的对象，即<code>theTree</code>。 然后打印<code>theTree</code>对象的<code>constructor</code>属性。</p>
<pre><code class="language-js">function Tree(name) {
   this.name = name;
}

var theTree = new Tree("Redwood");
console.log( "theTree.constructor is " + theTree.constructor );</code></pre>
<p>打印输出：</p>
<pre><code class="language-js">theTree.constructor is function Tree(name) {
    this.name = name;
}</code></pre>
<h3 id="Example:_Changing_the_constructor_of_an_object" name="Example:_Changing_the_constructor_of_an_object">改变对象的 constructor</h3>
<p>下面的例子展示了如何修改基本类型对象的 <code>constructor</code> 属性的值。只有 <code>true</code>, <code>1</code> 和 <code>"test"</code> 的不受影响，因为创建他们的是只读的原生构造函数（native constructors）。这个例子也说明了依赖一个对象的 <code>constructor</code> 属性并不安全。</p>
<pre><code class="language-js">function Type() { };

var	types = [
	new Array,
    [],
	new Boolean,
    true,        // remains unchanged
	new Date,
	new Error,
	new Function,
	function(){},
	Math,	
	new Number,
	1,           // remains unchanged
	new Object,
	{},
	new RegExp,
	/(?:)/,
	new String,
	"test"       // remains unchanged
];

for(var i = 0; i &lt; types.length; i++) {
	types[i].constructor = Type;
	types[i] = [ types[i].constructor, types[i] instanceof Type, types[i].toString() ];
};

console.log( types.join("\n") );
</code></pre>
<p>此示例显示以下输出：</p>
<pre><code  class="language-javascript">function Type() {},false,
function Type() {},false,
function Type() {},false,false
function Boolean() {
    [native code]
},false,true
function Type() {},false,Mon Sep 01 2014 16:03:49 GMT+0600
function Type() {},false,Error
function Type() {},false,function anonymous() {

}
function Type() {},false,function () {}
function Type() {},false,[object Math]
function Type() {},false,0
function Number() {
    [native code]
},false,1
function Type() {},false,[object Object]
function Type() {},false,[object Object]
function Type() {},false,/(?:)/
function Type() {},false,/(?:)/
function Type() {},false,
function String() {
    [native code]
},false,test
</code></pre>
<p> </p>
<h3 id="改变函数的_constructor">改变函数的 constructor</h3>
<p>大多数情况下，此属性用于定义一个构造函数，并使用<strong>new</strong>和继承原型链进一步调用它。</p>
<pre><code  class="language-javascript">function Parent() {}
Parent.prototype.parentMethod = function parentMethod() {};

function Child() {}
Child.prototype = Object.create(Parent.prototype); // re-define child prototype to Parent prototype

Child.prototype.constructor = Child; // return original constructor to Child</code></pre>
<p>但为什么我们需要在这里执行最后一行？很不幸正确答案是 - 看情况而定。</p>
<p>让我们尝试定义在哪些情况下重新分配原始构造函数将起主要作用，何时它将是一个额外的未使用的代码行。</p>
<p>试想下一种情况：该对象具有创建自身的<strong>create</strong>方法。</p>
<pre><code  class="language-javascript">function Parent() {};
function CreatedConstructor() {}

CreatedConstructor.prototype = Object.create(Parent.prototype);

CreatedConstructor.prototype.create = function create() {
  return new this.constructor();
}

new CreatedConstructor().create().create(); // error undefined is not a function since constructor === Parent
</code></pre>
<p>在上面的示例中，将显示异常，因为构造函数链接到Parent。</p>
<p>为了避免它，只需分配您将要使用的必要构造函数。</p>
<pre><code  class="language-javascript">function Parent() {}; 
function CreatedConstructor() {} 

CreatedConstructor.prototype = Object.create(Parent.prototype); 
CreatedConstructor.prototype.constructor = CreatedConstructor; // set right constructor for further using

CreatedConstructor.prototype.create = function create() { 
  return new this.constructor();
} 

new CreatedConstructor().create().create(); // it's pretty fine</code></pre>
<p>好的，现在很清楚为什么更改构造函数会很有用。</p>
<p>让我们再考虑一个案例。</p>
<pre><code  class="language-javascript">function ParentWithStatic() {}

ParentWithStatic.startPosition = { x: 0, y:0 };
ParentWithStatic.getStartPosition = function getStartPosition() {
  return this.startPosition;
} 

function Child(x, y) {
  this.position = {
    x: x,
    y: y
  };
}

Child.prototype = Object.create(ParentWithStatic.prototype); 
Child.prototype.constructor = Child;

Child.prototype.getOffsetByInitialPosition = function getOffsetByInitialPosition() {
  var position = this.position;
  var startPosition = this.constructor.getStartPosition(); // error undefined is not a function, since the constructor is Child

  return {
    offsetX: startPosition.x - position.x,
    offsetY: startPosition.y - position.y
  }
};</code></pre>
<p>对于此示例，我们需要保持父构造函数继续正常工作。</p>
<p><strong>总结</strong>：手动设置或更新构造函数可能会导致不同且有时令人困惑的后果。为了防止它，只需在每个特定情况下定义构造函数的角色。在大多数情况下，不使用构造函数，并且不需要重新分配构造函数。</p>
<h2 id="规范" style="margin-bottom: 20px; line-height: 30px;">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf" hreflang="en" lang="en" rel="noopener" title="ECMAScript 1st Edition (ECMA-262)">ECMAScript 1st Edition (ECMA-262)</a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition. Implemented in JavaScript 1.1.</td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.2.4.1" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Object.prototype.constructor</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-object.prototype.constructor" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Object.prototype.constructor</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-object.prototype.constructor" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Object.prototype.constructor</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容">浏览器兼容</h2>
<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>
<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>constructor</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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              1</td><td class="bc-supports-yes bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-webview_android"><span class="bc-browser-name">WebView Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              4</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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              Yes</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">
<abbr class="bc-level bc-level-yes only-icon" title="Full support">
<span>Full support</span>
                  
                </abbr></span></dt><dd>Full support</dd></dl></section></div><p></p>
</article>