JavaScript 如何检测函数是否被调用为构造函数

问题是要确定函数调用是否是构造函数调用。

方法1

使用 instanceof 属性 。
如果当前实例是函数的实例，那么这是一个构造函数调用。
否则，这是一个普通的函数调用。

示例： 这个示例实现了上述方法。

<h1 style="color:green;"> 
    GeeksforGeeks 
</h1> 
  
<p id="GFG_UP"></p> 
  
<button onclick="gfg_Run()"> 
    Click here 
</button> 
  
<p id="GFG_DOWN"></p> 
  
<script> 
    var el_up = document.getElementById("GFG_UP"); 
    var el_down = document.getElementById("GFG_DOWN"); 
      
    el_up.innerHTML = "Click on the button to check if the " 
                + "function is called as a constructor<br>" 
                + "Function name - GFG_FUN2"; 
      
    function GFG_FUN2(val) { 
        var temp = false; 
          
        if (this instanceof GFG_FUN2 && 
                !this.__previouslyConstructedByVal) { 
            temp = true; 
            this.__previouslyConstructedByVal = true; 
        } 
          
        return temp; 
    } 
      
    function gfg_Run() { 
          
        // Function call 
        var temp = GFG_FUN2(); 
          
        if (temp == true) { 
            el_down.innerHTML = 
                    "Function is called as constructor."; 
        } 
        else { 
            el_down.innerHTML = 
                    "Function is not called as constructor."; 
        } 
    } 
</script>

输出：

JavaScript 如何检测函数是否被调用为构造函数

方法2

使用 . 构造器属性 .
如果 this.constructor 等于函数名，则为构造函数调用。
否则，为普通函数调用。

示例： 这个示例演示了上面讨论的方法。

<h1 style="color:green;"> 
    GeeksforGeeks 
</h1> 
<p id="GFG_UP"></p> 
<button onclick="gfg_Run()"> 
    Click here 
</button> 
<p id="GFG_DOWN"></p> 
<script> 
    var el_up = document.getElementById("GFG_UP"); 
    var el_down = document.getElementById("GFG_DOWN"); 
    el_up.innerHTML = "Click on the button to check if the " + 
                    "function is called as a constructor<br>" + 
                    "Function name - GFG_FUN2"; 
      
    function GFG_FUN2(val) { 
        var temp = false; 
        if (this.constructor == GFG_FUN2) { 
            temp = true; 
        } 
        if (temp == true) { 
            el_down.innerHTML = "Function is called as constructor."; 
        } else { 
            el_down.innerHTML = "Function is not called as constructor."; 
        } 
    } 
      
    function gfg_Run() { 
        new GFG_FUN2(); //Call to the function. 
    } 
</script>