（1）全局环境
在全局环境使用this，它指的就是顶层对象window。
<span style="font-size: 16px; font-family: "Microsoft YaHei""><code><span class="k">this <span class="o">=== <span class="nb">window <span class="c1">// true<span class="kd">function <span class="nx">f<span class="p">() <span class="p">{ <span class="nx">console<span class="p">.<span class="nx">log<span class="p">(<span class="k">this <span class="o">=== <span class="nb">window<span class="p">); <span class="c1">// true<span class="p">}<br/>不管是不是在函数内部，只要是在全局环境下运行，<code class="highlighter-rouge">this</code>就是指顶层对象<code class="highlighter-rouge">window</code>。<br/></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span>（2）构造函数
构造函数中的this，指的是实例对象。
<span style="font-size: 16px; font-family: "Microsoft YaHei""><code><span class="kd">var <span class="nx">Obj <span class="o">= <span class="kd">function <span class="p">(<span class="nx">p<span class="p">) <span class="p">{ <span class="k">this<span class="p">.<span class="nx">p <span class="o">= <span class="nx">p<span class="p">;<span class="p">};<span class="nx">Obj<span class="p">.<span class="nx">prototype<span class="p">.<span class="nx">m <span class="o">= <span class="kd">function<span class="p">() <span class="p">{ <span class="k">return <span class="k">this<span class="p">.<span class="nx">p<span class="p">;<span class="p">};</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span><br/><br/><span style="font-size: 16px; font-family: "Microsoft YaHei"">上面代码定义了一个构造函数<code class="highlighter-rouge">Obj</code>。由于<code class="highlighter-rouge">this</code>指向实例对象，所以在构造函数内部定义<code class="highlighter-rouge">this.p</code>，就相当于定义实例对象有一个<code class="highlighter-rouge">p</code>属性；然后<code class="highlighter-rouge">m</code>方法可以返回这个p属性。</span>（3）对象的方法
当A对象的方法被赋予B对象，该方法中的this就从指向A对象变成了指向B对象。所以要特别小心，将某个对象的方法赋值给另一个对象，会改变this的指向。
<span style="font-size: 16px; font-family: "Microsoft YaHei""><code><span class="kd">var <span class="nx">obj <span class="o">=<span class="p">{ <span class="na">foo<span class="p">: <span class="kd">function <span class="p">() <span class="p">{<span class="nx">console<span class="p">.<span class="nx">log<span class="p">(<span class="k">this<span class="p">); <span class="p">}<span class="p">};<span class="nx">obj<span class="p">.<span class="nx">foo<span class="p">() <span class="c1">// obj</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span><br/><br/>obj.foo方法执行时，它内部的this指向obj。
但是，只有这一种用法（直接在obj对象上调用foo方法），this指向obj；其他用法时，this都指向代码块当前所在对象（浏览器为window对象）。
// 情况一(obj.foo = obj.foo)() // window// 情况二(false || obj.foo)() // window// 情况三(1, obj.foo)() // window
obj.foo先运算再执行，即使它的值根本没有变化，this也不再指向obj了。
可以这样理解，在JavaScript引擎内部，obj和obj.foo储存在两个内存地址，简称为M1和M2。只有obj.foo()这样调用时，是从M1调用M2，因此this指向obj。但是，上面三种情况，都是直接取出M2进行运算，然后就在全局环境执行运算结果（还是M2），因此this指向全局环境。
<span style="font-size: 16px; font-family: "Microsoft YaHei""><code><span class="c1">// 情况一<span class="p">(<span class="nx">obj<span class="p">.<span class="nx">foo <span class="o">= <span class="kd">function <span class="p">() <span class="p">{ <span class="nx">console<span class="p">.<span class="nx">log<span class="p">(<span class="k">this<span class="p">);<span class="p">})()<span class="c1">// 情况二<span class="p">(<span class="kc">false <span class="o">|| <span class="kd">function <span class="p">() <span class="p">{ <span class="nx">console<span class="p">.<span class="nx">log<span class="p">(<span class="k">this<span class="p">);<span class="p">})()<span class="c1">// 情况三<span class="p">(<span class="mi">1<span class="p">, <span class="kd">function <span class="p">() <span class="p">{ <span class="nx">console<span class="p">.<span class="nx">log<span class="p">(<span class="k">this<span class="p">);<span class="p">})()</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span><br/><br/><span style="font-size: 16px; font-family: "Microsoft YaHei"">同样的，如果某个方法位于多层对象的内部，这时为了简化书写，把该方法赋值给一个变量，往往会得到意料之外的结果。</span>var a = { b: {m: function() { console.log(this.p);},p: 'Hello' }};var hello = a.b.m;hello() // undefined上面代码中，m是多层对象内部的一个方法。为求简便，将其赋值给hello变量，结果调用时，this指向了顶层对象。为了避免这个问题，可以只将m所在的对象赋值给hello，这样调用时，this的指向就不会变。
<span style="font-size: 16px; font-family: "Microsoft YaHei""><code><span class="kd">var <span class="nx">hello <span class="o">= <span class="nx">a<span class="p">.<span class="nx">b<span class="p">;<span class="nx">hello<span class="p">.<span class="nx">m<span class="p">() <span class="c1">// Hello</span></span></span></span></span></span></span></span></span></span></span></span></code></span><br/><br/>
（4）Node
在Node中，this的指向又分成两种情况。全局环境中，this指向全局对象global；模块环境中，this指向module.exports。
<span style="font-size: 16px; font-family: "Microsoft YaHei""><code><span class="c1">// 全局环境<span class="k">this <span class="o">=== <span class="nx">global <span class="c1">// true<span class="c1">// 模块环境<span class="k">this <span class="o">=== <span class="nx">module<span class="p">.<span class="nx">exports <span class="c1">// true</span></span></span></span></span></span></span></span></span></span></span></span></code></span><br/><br/><span style="font-size: 16px; font-family: "Microsoft YaHei""><br/></span><br/><br/>
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