How to Replace First 4 Characters in Javascript

In this tutorial, you will learn how to replace first 4 characters in javascript. The length of a string depends upon the number of characters and it can have 4 or more characters in it. These characters can be letters, numbers, or special characters. It can also be a combination of letters, numbers, and special characters.

There are numerous ways to replace the first 4 characters of string. We are going to create a custom function that involves the usage of for loop to loop through all characters. Later, we will use replace() method to replace the first 4 characters with an exclamation mark (!).

In the following example, we have one global variable that holds a string. Upon click of a button, we will replace the first 4 characters of the string and display the result on the screen.  Please have a look over the code example and the steps given below.


  • 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 “Get” 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">

  <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">


  <div class="container">    

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

.container {        
    text-align: center;

button {
  margin-top: 10px;
  padding: 10px 20px;


  • We have selected the button element and h1 element using the document.querySelector() method and stored them in btnGet 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.
  • In the event handler function, we are calling the replaceCharacters() method and passing myString and 4 as parameters. This method will be responsible for replacing the first 4 characters and returning the new string.
  • In the replaceCharacters() method, we are using for loop to loop through all the characters. Inside the loop, we are using replace() method to replace the first 4 characters with an exclamation mark (!).
  • The final string is stored in the newStr variable.
  • We are getting out of the loop using the break statement after replacing the first 4 characters.
  • The returned string is stored in the result variable and we are displaying the result in the h1 element using the innerText property.
let btnGet = document.querySelector("button");
let output = document.querySelector("h1");

let myString = "Hello world";

btnGet.addEventListener("click", () => {
  let result = replaceCharacters(myString, 4);
  output.innerText = result;

function replaceCharacters(str, num) {
  let newStr = str;
  let count = 0;
  for (const char of str) {
    newStr = newStr.replace(char, "!");
    if (count == num) break;
  return newStr;