JavaScript Program to Check If a Variable is of Function Type

In this example, you will learn to write a JavaScript program that will check if a variable is of function type.

To understand this example, you should have the knowledge of the following JavaScript programming topics:


Example 1: Using instanceof Operator

// program to check if a variable is of function type

function testVariable(variable) {
      
    if(variable instanceof Function) {
        console.log('The variable is of function type');
    }
    else {
        console.log('The variable is not of function type');
    }
}

const count = true;
const x = function() {
    console.log('hello')
};

testVariable(count);
testVariable(x);

Output

The variable is not of function type
The variable is of function type

In the above program, the instanceof operator is used to check the type of variable.


Example 2: Using typeof Operator

// program to check if a variable is of function type

function testVariable(variable) {
      
    if(typeof variable === 'function') {
        console.log('The variable is of function type');
    }
    else {
        console.log('The variable is not of function type');
    }
}

const count = true;
const x = function() {
    console.log('hello')
};

testVariable(count);
testVariable(x);

Output

The variable is not of function type
The variable is of function type

In the above program, the typeof operator is used with strict equal to === operator to check the type of variable.

The typeof operator gives the variable data type. === checks if the variable is equal in terms of value as well as the data type.


Example 3: Using Object.prototype.toString.call() Method

// program to check if a variable is of function type

function testVariable(variable) {
      
    if(Object.prototype.toString.call(variable) == '[object Function]') {
        console.log('The variable is of function type');
    }
    else {
        console.log('The variable is not of function type');
    }
}

const count = true;
const x = function() {
    console.log('hello')
};

testVariable(count);
testVariable(x);

Output

The variable is not of function type
The variable is of function type