These methods create new strings by combining or modifying content. Remember, strings are immutable—these methods return new strings.

concat()

Concatenates strings together:

'Flavio'.concat(' ').concat('Copes') //'Flavio Copes'

You can specify multiple arguments:

'Flavio'.concat(' ', 'Copes') //'Flavio Copes'

Joining Strings with + Operator

The simplest and fastest way to join strings is using the + operator:

const name = 'Flavio'
const surname = 'Copes'
const fullname = name + ' ' + surname //'Flavio Copes'

Or use += to append to an existing string:

let name = 'Flavio'
name += ' Copes' //'Flavio Copes'

The + operator is generally recommended over concat() for simplicity and performance.

replace()

Finds the first occurrence of a pattern and replaces it. Returns a new string:

'JavaScript'.replace('Java', 'Type') //'TypeScript'

You can use a regular expression as the pattern:

'JavaScript'.replace(/Java/, 'Type') //'TypeScript'

replace() only replaces the first match unless you use the global flag (/g):

'JavaScript JavaX'.replace(/Java/g, 'Type') //'TypeScript TypeX'

The second parameter can be a function that receives the match details:

'JavaScript'.replace(/Java/, (match, index, originalString) => {
  console.log(match, index, originalString)
  return 'Test'
}) //TestScript

With capturing groups, the captured values are passed as additional arguments:

'2015-01-02'.replace(
  /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/,
  (match, year, month, day, index, originalString) => {
    console.log(match, year, month, day, index, originalString)
    return 'Test'
  }
) //Test

Replacing All Occurrences

Using a regular expression with the global flag:

const phrase = 'I love my dog! Dogs are great'
phrase.replace(/dog/g, 'cat') //"I love my cat! Dogs are great"

For case-insensitive replacement, add the i flag:

phrase.replace(/dog/gi, 'cat') //"I love my cat! cats are great"

If the string contains special regex characters, escape them:

const escapeRegExp = (string) => {
  return string.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
}

Alternative: split and join

const phrase = 'I love my dog! Dogs are great'
phrase.split('dog').join('cat') //"I love my cat! Dogs are great"

repeat()

Repeats a string the specified number of times:

'Ho'.repeat(3) //'HoHoHo'

Returns an empty string if the parameter is 0 or omitted. Throws a RangeError if the parameter is negative.

Introduced in ES2015.

Lessons in this unit:

0:	Introduction
1:	String Basics
2:	Accessing Characters
3:	Searching Strings
4:	Extracting Substrings
5:	Transforming Strings
6:	▶︎ Modifying Strings
7:	Trimming and Padding
8:	String Recipes
9:	Unicode and UTF-8
10:	Printable ASCII characters list
11:	Non-printable ASCII characters list