How to detect if a function is called as constructor ?

The problem is to identify whether the function call is a constructor call or not. 

Approach 1:

• Use the instanceof property.
• If the current instance is the instance of a function then this is a constructor call.
• Else, this is a general function call.

Example: This example implements the above approach. 

html

<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>

Output:

How to detect if a function is called as constructor?

Approach 2:

• Use the .constructor property.
• If this.constructor is equal to the function name then this is a constructor call.
• Else, this is a general function call.

Example: This example illustrates the approach discussed above. 

html

<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>

Output:

How to detect if a function is called as constructor?