March 22, 2022

How to Check if Variable is File in Javascript

By Krishnaa JavaScript Questions 0 Comments

In this tutorial, you will learn how to check if variable is file in javascript. In HTML, to provide a user the ability to select a file, we make use of the input element which is of file type. We can select a single file as well as multiple files in a single go. Files can be of different types and using a File object, we can get details about any file in javascript.

There are numerous ways to check if a variable is file or not. We are going to use one of the easiest solutions which involve the usage of the instanceof and ternary (?) operators. The ternary operator is also known as the conditional operator which acts similar to the if-else statement. The instanceof operator verifies whether the object is an instance of the specified type

In the following example, we have one global variable and upon click of a button, we will check if the variable is file or not 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 file object as its value.
We have attached a click event listener to the button element.

In the event handler function, we are using instanceof and ternary (?) operators to verify whether myVar is file or not. Depending upon the result of the check, we will assign “Yes” or “No” 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 = new File([00,01,02], "file.txt");

btnCheck.addEventListener("click", () => { 

  let result = myVar instanceof File ? "Yes" : "No";
  output.innerText = result;
  
});

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