March 24, 2022

How to Check if Variable is not Declared in Javascript

By Krishnaa JavaScript Questions 0 Comments

In this tutorial, you will learn how to check if variable is not declared in javascript. Before accessing a variable, it is extremely important to make sure that it is declared in the current context. Otherwise, you will get unforeseen errors during the compilation or runtime.

There are numerous ways to check if a variable is not declared. We are going to use one of the easiest solutions which involve the usage of the try-catch block and ternary (?) operator. The ternary operator is also known as the conditional operator which acts similar to the if-else statement. In the try block, we run a code that is going to be tested for errors. In the catch block, we execute a certain piece of code after an error is thrown.

In the following example, upon click of a button, we will check if a variable is not declared 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 attached a click event listener to the button element.
In the event handler function, the isDeclared variable is set to true initially. In the try block, we are trying to access the myVar variable. If myVar is declared, we will never enter into the catch block but if it does not, then we will get a Reference error.

We are using the if statement to verify whether the error is a Reference error or not. If yes, then we will set isDeclared to false.
We are using the ternary operator to check whether isDeclared is true or false. Depending upon the result of the check, we will assign “Declared” or “Undeclared“ 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");



btnCheck.addEventListener("click", () => {
  
  let isDeclared = true;
  try {
    myVar;
  } catch (e) {
    if (e.name == "ReferenceError") {
      isDeclared = false;
    }
  }

  let result = !isDeclared ? "Undeclared" : "Declared";
  output.innerText = result;
});

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