Inserting a String at a Specific Index in JavaScript

Last Updated : 8 Sep, 2026

In JavaScript, inserting a string at a specific index creates a new string by placing the new content between parts of the original string.

  • Strings remain unchanged because they are immutable.
  • Different methods can be used based on readability and requirements.
  • The result combines the original content with the inserted string at the desired position.

Approach 1: Using the slice() Method

The slice() method divides the original string into two parts at the specified index. The new string is placed between these two parts to create the updated string.

JavaScript 
let str = "GeeksGeeks";
let str2 = "For";
let idx = 5;

let res = str.slice(0, idx) + str2 + str.slice(idx);

console.log(res);

Approach 2: Using the substring() Method

The substring() method can also divide a string into two parts. The new string is inserted between the extracted portions.

JavaScript 
let str = "GeeksGeeks";
let str2 = "For";
let idx = 5;

let res = str.substring(0, idx) + str2 + str.substring(idx);

console.log(res);

Approach 3: Using a Regular Expression

A regular expression can insert a string after a matched group of characters using the replace() method.

JavaScript 
let str = "hello";

let res = str.replace(/(.{3})/, "$1***");

console.log(res);

Approach 4: Using Template Literals

Template literals provide a clean and readable way to combine different parts of a string with the new content.

JavaScript 
const str = "Hello, World!";
const str1 = "Amazing ";
const idx = 7;

const s1 = str.slice(0, idx);
const s2 = str.slice(idx);

const res = `${s1}${str1}${s2}`;

console.log(res);

Approach 5: Using the Array Spread Operator

This approach uses the Spread Operator to convert the string into an array of characters. The splice() method inserts the new string at the specified index, and join() converts the array back into a string.

JavaScript 
let str = "HelloWorld";
let str1 = "Beautiful";
let idx = 5;

let arr = [...str];

arr.splice(idx, 0, ...str1);

let res = arr.join("");

console.log(res);

Approach 6: Using Loops

Loops provide manual control over the insertion process by constructing a new string character by character.

JavaScript 
function insertAt(str, str1, idx) {
    if (idx < 0 || idx > str.length) {
        return str;
    }

    let res = "";

    for (let i = 0; i < str.length; i++) {
        if (i === idx) {
            res += str1;
        }

        res += str[i];
    }

    if (idx === str.length) {
        res += str1;
    }

    return res;
}

let str = "Hello World!";
let str1 = "JavaScript ";
let idx = 6;

console.log(insertAt(str, str1, idx));
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