Inserting string at position x of another string

var output = a.substring(0, position) + b + a.substring(position);

Edit: replaced .substr with .substring because .substr is now a legacy function (per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr)

You can add this function to string class

String.prototype.insert_at=function(index, string)
{   
  return this.substr(0, index) + string + this.substr(index);
}

so that you can use it on any string object:

var my_string = "abcd";
my_string.insertAt(1, "XX");

Maybe it's even better if you determine position using indexOf() like this:

function insertString(a, b, at)
{
    var position = a.indexOf(at); 

    if (position !== -1)
    {
        return a.substr(0, position) + b + a.substr(position);    
    }  

    return "substring not found";
}

then call the function like this:

insertString("I want apple", "an ", "apple");

Note, that I put a space after the "an " in the function call, rather than in the return statement.

Using ES6 string literals, would be much shorter:

const insertAt = (str, sub, pos) => `${str.slice(0, pos)}${sub}${str.slice(pos)}`;
    
console.log(insertAt('I want apple', ' an', 6)) // logs 'I want an apple'

Well just a small change 'cause the above solution outputs

"I want anapple"

instead of

"I want an apple"

To get the output as

"I want an apple"

use the following modified code

var output = a.substr(0, position) + " " + b + a.substr(position);

Quick fix! If you don't want to manually add a space, you can do this:

var a = "I want apple";
var b = "an";
var position = 6;
var output = [a.slice(0, position + 1), b, a.slice(position)].join('');
console.log(output);

(edit: i see that this is actually answered above, sorry!)

If ES2018's lookbehind is available, one more regexp solution, that makes use of it to "replace" at a zero-width position after the Nth character (similar to @Kamil Kie?czewski's, but without storing the initial characters in a capturing group):

"I want apple".replace(/(?<=^.{6})/, " an")

var a = "I want apple";
var b = " an";
var position = 6;

var r= a.replace(new RegExp(`(?<=^.{${position}})`), b);

console.log(r);
console.log("I want apple".replace(/(?<=^.{6})/, " an"));

var a = "I want apple";
var b = " an";
var position = 6;
var output = [a.slice(0, position), b, a.slice(position)].join('');
console.log(output);

Optional: As a prototype method of String

The following can be used to splice text within another string at a desired index, with an optional removeCount parameter.

if (String.prototype.splice === undefined) {
  /**
   * Splices text within a string.
   * @param {int} offset The position to insert the text at (before)
   * @param {string} text The text to insert
   * @param {int} [removeCount=0] An optional number of characters to overwrite
   * @returns {string} A modified string containing the spliced text.
   */
  String.prototype.splice = function(offset, text, removeCount=0) {
    let calculatedOffset = offset < 0 ? this.length + offset : offset;
    return this.substring(0, calculatedOffset) +
      text + this.substring(calculatedOffset + removeCount);
  };
}

let originalText = "I want apple";

// Positive offset
console.log(originalText.splice(6, " an"));
// Negative index
console.log(originalText.splice(-5, "an "));
// Chaining
console.log(originalText.splice(6, " an").splice(2, "need", 4).splice(0, "You", 1));

.as-console-wrapper { top: 0; max-height: 100% !important; }

The Underscore.String library has a function that does Insert

insert(string, index, substring) => string

like so

insert("Hello ", 6, "world");
// => "Hello world"

try

a.slice(0,position) + b + a.slice(position)

var a = "I want apple";
var b = " an";
var position = 6;

var r= a.slice(0,position) + b + a.slice(position);

console.log(r);

or regexp solution

"I want apple".replace(/^(.{6})/,"$1 an")

var a = "I want apple";
var b = " an";
var position = 6;

var r= a.replace(new RegExp(`^(.{${position}})`),"$1"+b);

console.log(r);
console.log("I want apple".replace(/^(.{6})/,"$1 an"));

var array = a.split(' '); 
array.splice(position, 0, b);
var output = array.join(' ');

This would be slower, but will take care of the addition of space before and after the an Also, you'll have to change the value of position ( to 2, it's more intuitive now)