`String`

The String global object converts any value to a string primitive when it’s called as a function, and its prototype holds the methods that every string can call. The primitive itself is the string type, and a method call such as "js".toUpperCase() reaches String.prototype through a temporary wrapper object:

> String(42)
'42'

> "realjs".toUpperCase()
'REALJS'

> typeof new String("hi")
'object'

String is an ECMAScript built-in. Literal syntax, typeof, coercion rules, and how .length counts UTF-16 code units are covered in the string entry.

Syntax

The String function takes one optional value to convert:

String([<value>])
new String([<value>])

Parameter	Description
`value`	An optional value of any type to convert. It defaults to the empty string `""` when omitted

Without the new keyword, the call returns a string primitive. With new, it returns a String object that wraps the primitive. The new keyword is optional, but it changes the result. Calling String() on a symbol returns a string such as "Symbol(id)", while new String() throws a TypeError for one.

Key Static Methods and Properties

Member	Kind	Description
`String.fromCharCode()`	Static method	Returns a string built from UTF-16 code units
`String.fromCodePoint()`	Static method	Returns a string built from Unicode code points, including those above `0xFFFF`
`String.raw()`	Static method	Returns the raw text of a template literal, with escape sequences left unprocessed

Key Instance Methods and Properties

Member	Kind	Description
`.length`	Instance property	Returns the number of UTF-16 code units in the string
`.at()`	Instance method	Returns a one-character string holding the UTF-16 code unit at an index, counting from the end for negative indexes (ES2022)
`.includes()`	Instance method	Checks whether the string contains a substring
`.startsWith()`, `.endsWith()`	Instance method	Check whether the string begins or ends with a substring
`.indexOf()`, `.lastIndexOf()`	Instance method	Return the position of a substring, or `-1` when it’s missing
`.slice()`	Instance method	Returns a section of the string, with negative indexes counting from the end
`.split()`	Instance method	Returns an array of substrings divided at a separator
`.replace()`	Instance method	Returns a new string with the first match of a pattern replaced, or every match for a global regular expression
`.replaceAll()`	Instance method	Returns a new string with every match of a pattern replaced (ES2021)
`.match()`, `.matchAll()`	Instance method	Return the matches of a regular expression
`.trim()`, `.trimStart()`, `.trimEnd()`	Instance method	Return a new string with whitespace removed from one or both ends
`.padStart()`, `.padEnd()`	Instance method	Return a new string padded to a target length
`.toUpperCase()`, `.toLowerCase()`	Instance method	Return a new string with the case converted
`.repeat()`	Instance method	Returns the string repeated a given number of times
`.normalize()`	Instance method	Returns the Unicode normalization form of the string
`.localeCompare()`	Instance method	Compares two strings in locale-aware sort order
`.isWellFormed()`, `.toWellFormed()`	Instance method	Check for or replace lone surrogates (ES2024)

Strings are immutable, so every method returns a new value and leaves the original unchanged. .isWellFormed() and .toWellFormed() shipped in all major browsers in October 2023 and are now Baseline Widely available.

Examples

A signup form receives a username with stray spaces and mixed case. Chaining .trim() and .toLowerCase() cleans it up before storage:

> "  Ada.Lovelace  ".trim().toLowerCase()
'ada.lovelace'

An invoice system needs fixed-width order numbers. The .padStart() method fills the front with zeros:

> String(7).padStart(5, "0")
'00007'

An upload handler checks a file’s extension and grabs its last character. The .endsWith() and .at() methods do both without index arithmetic:

> const fileName = "report.pdf";
undefined

> fileName.endsWith(".pdf")
true
> fileName.at(-1)
'f'

A Windows path in a template literal would turn \t and \n into a tab and a newline. Tagging the literal with String.raw keeps the backslashes:

> String.raw`C:\temp\new`
'C:\\temp\\new'

Common Gotchas

Wrapper objects created with new String() compare by reference and are always truthy, even when they wrap an empty string:

> new String("a") === new String("a")
false

> Boolean(new String(""))
true

A string pattern passed to .replace() replaces only the first match:

> "2026-11-04".replace("-", "/")
'2026/11-04'

> "2026-11-04".replaceAll("-", "/")
'2026/11/04'

The .replaceAll() method, or a regular expression with the g flag, replaces every match.

A developer holds up a LOGIN tile beside a clay machine that splits FIX LOGIN BUG into words, slices and trims tiles, and joins them into FIX-LOGIN-BUG.

Course

JavaScript Strings for Pythonistas

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By Leodanis Pozo Ramos • Updated Nov. 7, 2026

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