Check if a Variable is of Function Type using JavaScript

Last Updated : 03 Oct, 2024

A function in JavaScript is a 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.

javascript

let 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:

Table of Content

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.

JavaScript

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

Output

Variable is of function type

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.

JavaScript

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

Output

Variable is of function type

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.

JavaScript

// Declare a variable and initialize it with an anonymous function let gfg = function () {     // A set of statements };  // Function to check if a variable is of function type or not function testing(x) {     if (typeof x === 'function') {         console.log("Variable is of function type");     } else {         console.log("Variable is not of function type");     } }  // Function call testing(gfg);