如何检测函数是否被称为构造函数？

问题是确定函数调用在哪里是构造函数调用。

方法一：

• 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，这是一个通用函数调用。

示例：此示例实现了上述方法。

  

    
        How to detect if a function is called
        as constructor in JavaScript ?
    

  

      
     
        GeeksForGeeks 
    
      
    
    
      
    
        Click here
    
      
    
    
      
    
        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.";
            }
        }
    

  

输出：

• 在点击按钮之前：
  
• 点击按钮后：
  

方法二：

• 使用.constructor属性。
• 如果this.constructor等于函数名，那么这是一个构造函数调用。
• 否则，这是一个通用函数调用。

示例：此示例说明了上面讨论的方法。

  

    
        Detect function is called as constructor
    

  

      
            GeeksforGeeks  
        
    
    
    
        Click here
    
    
    
    
        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.
        }
    

  

输出：

• 在点击按钮之前：
  
• 点击按钮后：