JavaScript - replace part of string from given index

In this article, we're going to have a look at how to replace part of a string from a given index in JavaScript.

Quick solution:

// ONLINE-RUNNER:browser;

var text = 'This is text.'
var index = 2;
var insert = '__';

var result = text.substring(0, index) + insert + text.substring(index + insert.length);

console.log(result);

Note: read this article to know how to remove part of text for given indexes.

Check the below example for a better solution.

Custom method example

Presented in this section solution allows inserting text outside of given string filling space with any characters (by default with space).

// ONLINE-RUNNER:browser;

function repeatText(count, text) {
	var result = '';
  	for (var i = 0; i < count; ++i) {
    	result += text;
    }
  	return result;
}

function replaceText(text, insert, index, spacer) {
  	if (spacer == null) {
      spacer = ' ';
    }
    var prefix = text.substring(0, index);
    var postfix = text.substring(index + insert.length);
  	var space = repeatText(index - text.length, spacer);
  
    return prefix + space + insert + postfix;
}

// Usage example:

console.log( replaceText('This is text.', '__',  0) ); // __is is text.
console.log( replaceText('This is text.', '__',  2) ); // Th__ is text.
console.log( replaceText('This is text.', '__', 11) ); // This is tex__

console.log( replaceText('This is text.', '__', 13) ); // This is text.__
console.log( replaceText('This is text.', '__', 15) ); // This is text.  __

console.log( replaceText('text...', '__', 10, '++') ); // text...++++++__

Custom method example (ES 2015)

ES 2015 introduced repeat() method for string.

// ONLINE-RUNNER:browser;

const replaceText = (text, insert, index, spacer = ' ') => {
  const prefix = text.substring(0, index);
  const postfix = text.substring(index + insert.length);
  const space = spacer.repeat(index < text.length ? 0 : index - text.length);

  return prefix + space + insert + postfix;
};

// Usage example:

console.log( replaceText('This is text.', '__',  0) ); // __is is text.
console.log( replaceText('This is text.', '__',  2) ); // Th__ is text.
console.log( replaceText('This is text.', '__', 11) ); // This is tex__

console.log( replaceText('This is text.', '__', 13) ); // This is text.__
console.log( replaceText('This is text.', '__', 15) ); // This is text.  __

console.log( replaceText('text...', '__', 10, '++') ); // text...++++++__