March 30, 2022

How to Check if String is Not Empty or Undefined in Javascript

By Krishnaa JavaScript Questions 0 Comments

In this tutorial, you will learn how to check if a string is not empty or undefined in javascript. Whenever you create a variable and do not assign any value to it, by default it is always undefined. It becomes defined only when you assign some value to it. A variable is considered to be empty when it holds an empty string as its value. An empty string has a length of 0.

There are numerous ways to check if a string is not empty or undefined. But for the sake of simplicity, we will use a strict equality operator (===), logical NOT(!) operator, typeof operator and ternary (?) operator to accomplish our goal. 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 that holds a string. Upon click of a button, we will check if the string is not empty or undefined 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 class="container">    
    <button>Check</button>
    <h1>Result</h1>
  </div>

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

</html>

.container {        
    text-align: center;
}

button {
  margin-top: 10px;
  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.
We have a global variable myString which holds an empty string as its value.
In the event handler function, we are using typeof operator, strict equality operator (===) and logical NOT (!) operator to check if myString is not empty or undefined. Depending upon the result of the check, we will assign “Valid” or “Invalid” 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 myString = "Hello World";

btnCheck.addEventListener("click", () => { 
  
  let result = myString !== "" && typeof myString !== "undefined" ? "Valid" : "Invalid";
  output.innerText = result;
  
});

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