January 14, 2022

How to Check if a String Starts with a Number in Javascript

By Krishnaa JavaScript Questions 0 Comments

In this tutorial, you will learn how to check if a string starts with a number in javascript. A string consists of multiple characters and these characters could be letters, numbers, or special characters. If a string starts with a number, then the first character will be a number between 0 and 9.

There are numerous ways to detect whether a string starts with a number or not. But for the sake of simplicity, we will use the regular expression and ternary operator (?) to accomplish our goal. The test() method of RegExpObject is used to perform a pattern search in a string and returns a Boolean value.

A string consists of multiple characters and these characters in a string have a 0-based index. That means the first character in a string has an index of 0 and to get that character by index, we can use charAt() method.

In the following example, we have one global variable that holds a string. Upon click of a button, we will check whether the string starts with a number 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 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 a string as its value.
We have the regExp variable which holds a regular expression /\d/ as its value.
In the event handler function, we are calling the charAt() method and passing 0 as a parameter to get the first character of the string.
We are calling the test() method of regExp and passing the first character of the string as a parameter. It will return a Boolean value which we are storing in the isMatch variable.
We are using the ternary operator (?) and checking whether isMatch is true or false. 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 myString = "2 Elephants";
let regExp = /\d/;


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

  let isMatch = regExp.test(myString.charAt(0))
  let result = isMatch ? "Yes" : "No";
  output.innerText = result;
  
});

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