March 23, 2022

How to Check if a Variable is a Function in Javascript

By Krishnaa JavaScript Questions 0 Comments

In this tutorial, you will learn how to check if a variable is a function in javascript. Function in javascript is just a block of code created to perform a particular task.

There are numerous ways to check if a variable is function. We are going to use one of the easiest solutions which involve the usage of the typeof and ternary (?) operators. The typeof operator returns the type of the variable. The ternary operator is also known as the conditional operator which acts similar to the if-else statement.

In the following example, we have one global variable and upon click of a button, we will check if the variable is function and display the result on the screen. Please have a look over the code example and the steps given below.

HTML & CSS

We have 3 elements in the HTML file (div, button, and h1). The div element is just a wrapper for the rest of the elements.
The innerText for the button element is “Check” and for the h1 element, it is “Result”.
We have done some basic styling using CSS and added the link to our style.css stylesheet inside the head element.
We have also included our javascript file script.js with a script tag at the bottom.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link rel="stylesheet" href="style.css">
    <title>Document</title>
</head>
<body>
    
    <div>
        <button>Check</button>
        <h1>Result</h1>
    </div>

    <script src="script.js"></script>
</body>
</html>

body {
    text-align: center;
}

div {
    display: inline-block;
}

button {
    display: inline-block;
    padding: 10px 20px;
}

Javascript

We have selected the button element and h1 element using the document.querySelector() method and stored them in btnCheck and output variables respectively.
We have a global variable myVar which holds a function as its value.
We have attached a click event listener to the button element.
In the event handler function, we are using typeof and ternary (?) operators to verify whether myVar is function or not. Depending upon the result of the check, we will assign “Assigned” or “Not Function” to the result variable
We are displaying the result in the h1 element using the innerText property.

let btnCheck = document.querySelector("button");
let output = document.querySelector("h1");

let myVar = function (){};
btnCheck.addEventListener("click", () => {
  let result = typeof myVar == 'function' ? "Function" : "Not Function";
  output.innerText = result;
});

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