Insert a string at a specific index

Insert a string at a specific index

How can I insert a string at a specific index of another string?
var txt1 = “foo baz”

Suppose I want to insert “bar ” after the “foo” how can I achieve that?
I thought of substring(), but there must be a simpler more straight forward way.


Solution 1:

You could prototype your own splice() into String.


if (!String.prototype.splice) {
     * {JSDoc}
     * The splice() method changes the content of a string by removing a range of
     * characters and/or adding new characters.
     * @this {String}
     * @param {number} start Index at which to start changing the string.
     * @param {number} delCount An integer indicating the number of old chars to remove.
     * @param {string} newSubStr The String that is spliced in.
     * @return {string} A new string with the spliced substring.
    String.prototype.splice = function(start, delCount, newSubStr) {
        return this.slice(0, start) + newSubStr + this.slice(start + Math.abs(delCount));


String.prototype.splice = function(idx, rem, str) {
    return this.slice(0, idx) + str + this.slice(idx + Math.abs(rem));

var result = "foo baz".splice(4, 0, "bar ");

document.body.innerHTML = result; // "foo bar baz"

EDIT: Modified it to ensure that rem is an absolute value.

Solution 2:

Inserting at a specific index (rather than, say, at the first space character) has to use string slicing/substring:

var txt2 = txt1.slice(0, 3) + "bar" + txt1.slice(3);

Solution 3:

Try this.
Here is a method I wrote that behaves like all other programming langauges.

String.prototype.insert = function (index, string) {
  if (index > 0)
    return this.substring(0, index) + string + this.substring(index, this.length);

  return string + this;

Example of use:

var something = "How you?";
something = something.insert(3, " are");



Solution 4:

Just make the following function:

function insert(str, index, value) {
    return str.substr(0, index) + value + str.substr(index);

and then use it like that:

alert(insert("foo baz", 4, "bar "));

Output: foo bar baz

It behaves exactly, like the C# (Sharp) String.Insert(int startIndex, string value).

NOTE: This insert function inserts the string value (third parameter) before the specified integer index (second parameter) in the string str (first parameter), and then returns the new string without changing str!

Solution 5:

UPDATE 2016: Here is another just-for-fun (but more serious!) prototype function based on one-liner RegExp approach (with prepend support on undefined or negative index):

 * Insert `what` to string at position `index`.
String.prototype.insert = function(what, index) {
    return index > 0
        ? this.replace(new RegExp('.{' + index + '}'), '$&' + what)
        : what + this;

console.log( 'foo baz'.insert('bar ', 4) );  // "foo bar baz"
console.log( 'foo baz'.insert('bar ')    );  // "bar foo baz"

Previous (back to 2012) just-for-fun solution:

var index = 4,
    what  = 'bar ';

'foo baz'.replace(/./g, function(v, i) {
    return i === index - 1 ? v + what : v;
});  // "foo bar baz"

Solution 6:

This is basically doing what @Bass33 is doing except I’m also giving the option of using a negative index to count from the end. Kind of like the substr method allows.

// use a negative index to insert relative to the end of the string.

String.prototype.insert = function (index, string) {
  var ind = index < 0 ? this.length + index  :  index;
  return  this.substring(0, ind) + string + this.substring(ind, this.length);

Use case:
Lets say you have full size images using a naming convention but can’t update the data to also provide thumbnail urls.

var url = '/images/myimage.jpg';
var thumb = url.insert(-4, '_thm');

//    result:  '/images/myimage_thm.jpg'