Inserting a String at a Specific Index in JavaScript

Last Updated : 25 Feb, 2026

In JavaScript, you can insert a string at a specific index by combining parts of the original string with the new string. This is useful when modifying text dynamically without overwriting existing content.

  • Use slice() to split the original string into two parts at the desired index.
  • Concatenate the first part, the new string, and the remaining part to form the updated string.
  • Since strings are immutable in JavaScript, this operation returns a new modified string.

These are the following methods to insert a string at a specific index in JavaScript:

1. Using the slice() Method

  • Inserting a string using the slice() method involves splitting the original string into two parts: one before the insertion point and one after.
  • The new string is then placed between these slices, effectively inserting it at the desired index.
JavaScript
let str = "GeeksGeeks";
let str2 = "For";
let idx = 5;
let res = str.slice(0, idx)
    + str2 + str.slice(idx);
console.log(res);

2. Using JavaScript substring() Method

Inserting a string using the substring() method involves splitting the original string into two parts based on the index, inserting the new string between these parts, and then concatenating them. This modifies the string with the desired insertion at the specified position.

JavaScript
let str = "GeeksGeeks";
let str2 = "For";
let idx = 5;
let res = str.substring(0, idx) + str2 + str.substring(idx);
console.log(res);

3. Using Regular Expression

Inserting a string using a Regular Expression involves matching a specific position or pattern in the original string and using the replace() method to insert the new string at the desired location within the matched pattern.

JavaScript
let str = 'hello';
let res = str.replace(/(.{3})/, '$1***');
console.log(res);

5. Using Template Literals

  • Template literals provide a straightforward and readable way to insert a new string at a specified index.
  • By breaking the original string into two parts and using template literals, you can easily insert the new string.
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);

6. Using Array Spread Operator

In this approach, we will convert the string to an array of characters, use the array spread operator to insert the new string at the desired index, and then join the array back into a single string. This method is efficient and leverages modern JavaScript syntax for clarity and conciseness.

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

let a = [...str];
a.splice(idx, 0, ...str1);
let res = a.join('');

console.log(res);

7. String Insertion Using Loops

  • Inserting a string using loops involves iterating through the original string until the specified index is reached, then adding the new string.
  • The loop continues with the remaining part of the original string, constructing a new string with the inserted content.
JavaScript
function insertAt(str, str1, idx) {
    if (idx < 0 || idx > str.length) {
        return str; // or throw an error
    }

    let res = '';
    let i = 0;
    while (i < idx) {
        res += str[i];
        i++;
    }
    let j = 0;
    while (j < str1.length) {
        res += str1[j];
        j++;
    }
    while (i < str.length) {
        res += str[i];
        i++;
    }

    return res;
}

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

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