How to check a variable is of function type using JavaScript ?

A function in JavaScript is the set of statements used to perform a specific task. A function can be either a named one or an anonymous one. The set of statements inside a function is executed when the function is invoked or called. A function can be assigned to a variable or passed to a method.

var gfg = function(){/* A set of statements */};

Here, an anonymous function is assigned to the variable named as ‘gfg’. There are various methods to check the variable is of function type or not. Some of them are discussed below:

1. Using instanceof operator: The instanceof operator checks the type of an object at run time. It return a corresponding boolean value, i.e, either true or false to indicate if the object is of a particular type or not.

   Example: This example uses instanceof operator to check a variable is of function type or not.

   <script>
   
   // Declare a variable and initialize it
   // with anonymous function
   var gfg = function(){/* A set of statements */};
   
   // Function to check a variable is of
   // function type or not
   function testing(x) {
   
       if(x instanceof Function) {
           document.write("Variable is of function type");
       }
       else {
           document.write("Variable is not of function type");
       }
   }
   
   // Function call
   testing(gfg);
   
   </script>

   Output:

   Variable is of function type

2. Using Strict Equal (===) operator: In JavaScript, ‘===’ Operator is used to check whether two entities are of equal values as well as of equal type provides a boolean result. In this example, we use the ‘===’ operator. This operator, called the Strict Equal operator, checks if the operands are of the same type.

   Example: This example uses === operator to check a variable is of function type or not.

   <script>
   
   // Declare a variable and initialize it
   // with anonymous function
   var gfg = function(){/* A set of statements */};
   
   // Function to check a variable is of
   // function type or not
   function testing(x)
   {
       if (typeof x === "function") {
           document.write("Variable is of function type");
       }
       else {
           document.write("Variable is not of function type");
       }
   }
   
   // Function call
   testing(gfg);
   
   </script>

   Output:

   Variable is of function type

3. Using object.prototype.toString: This method uses object.prototype.toString. Every object has a toString() method, which is called implicitly when a value of String type is expected. If the toString() method is not overridden, by default it returns ‘[object type]’ where ‘type’ is the object type.

   Example: This example uses object.prototype.toString operator to check a variable is of function type or not.

   <script>
   
   // Declare a variable and initialize it
   // with anonymous function
   var gfg = function(){/* A set of statements */};
   
   // Function to check a variable is of
   // function type or not
   function testing(x)
   {
       if (Object.prototype.toString.call(x) == '[object Function]')
       {
           document.write("Variable is of function type");
   
       }
       else {
           document.write("Variable is not of function type");
       }
   }
   
   // Function call
   testing(gfg);
   
   </script>

   Output:

   Variable is of function type