String
The String
object is used to represent and manipulate a
sequence of characters.
Description
Strings are useful for holding data that can be represented in text form. Some of the
most-used operations on strings are to check their length
, to build and concatenate them using the
+ and += string operators,
checking for the existence or location of substrings with the
indexOf()
method, or extracting substrings
with the substring()
method.
Creating strings
Strings can be created as primitives, from string literals, or as objects, using the
String()
constructor:
const string1 = "A string primitive";
const string2 = 'Also a string primitive';
const string3 = `Yet another string primitive`;
const string4 = new String("A String object");
String primitives and string objects can be used interchangeably in most situations. See "String primitives and String objects" below.
String literals can be specified using single or double quotes, which are treated identically, or using the backtick character `. This last form specifies a template literal: with this form you can interpolate expressions.
Character access
There are two ways to access an individual character in a string. The first is the
charAt()
method:
return 'cat'.charAt(1) // returns "a"
The other way (introduced in ECMAScript 5) is to treat the string as an array-like object, where individual characters correspond to a numerical index:
return 'cat'[1] // returns "a"
When using bracket notation for character access, attempting to delete or assign a
value to these properties will not succeed. The properties involved are neither writable
nor configurable. (See Object.defineProperty()
for more information.)
Comparing strings
In C, the strcmp()
function is used for comparing strings. In JavaScript,
you just use the less-than and greater-than operators:
let a = 'a'
let b = 'b'
if (a < b) { // true
console.log(a + ' is less than ' + b)
} else if (a > b) {
console.log(a + ' is greater than ' + b)
} else {
console.log(a + ' and ' + b + ' are equal.')
}
A similar result can be achieved using the localeCompare()
method inherited by String
instances.
Note that a == b
compares the strings in a
and
b
for being equal in the usual case-sensitive way. If you wish
to compare without regard to upper or lower case characters, use a function similar to
this:
function isEqual(str1, str2)
{
return str1.toUpperCase() === str2.toUpperCase()
} // isEqual
Upper case is used instead of lower case in this function, due to problems with certain UTF-8 character conversions.
String primitives and String objects
Note that JavaScript distinguishes between String
objects and
primitive string values. (The same is true of
Boolean
and Numbers
.)
String literals (denoted by double or single quotes) and strings returned from
String
calls in a non-constructor context (that is, called without using
the new
keyword) are primitive strings. JavaScript
automatically converts primitives to String
objects, so that it's possible
to use String
object methods for primitive strings. In contexts where a
method is to be invoked on a primitive string or a property lookup occurs, JavaScript
will automatically wrap the string primitive and call the method or perform the property
lookup.
let s_prim = 'foo'
let s_obj = new String(s_prim)
console.log(typeof s_prim) // Logs "string"
console.log(typeof s_obj) // Logs "object"
String primitives and String
objects also give different results when
using eval()
. Primitives passed to
eval
are treated as source code; String
objects are treated as
all other objects are, by returning the object. For example:
let s1 = '2 + 2' // creates a string primitive
let s2 = new String('2 + 2') // creates a String object
console.log(eval(s1)) // returns the number 4
console.log(eval(s2)) // returns the string "2 + 2"
For these reasons, the code may break when it encounters String
objects
when it expects a primitive string instead, although generally, authors need not worry
about the distinction.
A String
object can always be converted to its primitive counterpart with
the valueOf()
method.
console.log(eval(s2.valueOf())) // returns the number 4
Escape sequences
Special characters can be encoded using escape sequences:
Escape sequence | Unicode code point |
---|---|
\0 |
null character (U+0000 NULL) |
\' |
single quote (U+0027 APOSTROPHE) |
\" |
double quote (U+0022 QUOTATION MARK) |
\\ |
backslash (U+005C REVERSE SOLIDUS) |
\n |
newline (U+000A LINE FEED; LF) |
\r |
carriage return (U+000D CARRIAGE RETURN; CR) |
\v |
vertical tab (U+000B LINE TABULATION) |
\t |
tab (U+0009 CHARACTER TABULATION) |
\b |
backspace (U+0008 BACKSPACE) |
\f |
form feed (U+000C FORM FEED) |
\uXXXX …where XXXX is exactly 4 hex digits in the range 0000 –FFFF ; e.g., \u000A is the same as \n (LINE FEED); \u0021 is "! " |
Unicode code point between U+0000 and U+FFFF (the Unicode Basic Multilingual Plane) |
\u{X} …\u{XXXXXX} …where X …XXXXXX is 1–6 hex digits in the range 0 –10FFFF ; e.g., \u{A} is the same as \n (LINE FEED); \u{21} is "! " |
Unicode code point between U+0000 and U+10FFFF (the entirety of Unicode) |
\xXX …where XX is exactly 2 hex digits in the range 00 –FF ; e.g., \x0A is the same as \n (LINE FEED); \x21 is "! " |
Unicode code point between U+0000 and U+00FF (the Basic Latin and Latin-1 Supplement blocks; equivalent to ISO-8859-1) |
Long literal strings
Sometimes, your code will include strings which are very long. Rather than having lines that go on endlessly, or wrap at the whim of your editor, you may wish to specifically break the string into multiple lines in the source code without affecting the actual string contents. There are two ways you can do this.
Method 1
You can use the + operator to append multiple strings together, like this:
let longString = "This is a very long string which needs " +
"to wrap across multiple lines because " +
"otherwise my code is unreadable."
Method 2
You can use the backslash character (\
) at the end of each line to
indicate that the string will continue on the next line. Make sure there is no space or
any other character after the backslash (except for a line break), or as an indent;
otherwise it will not work.
That form looks like this:
let longString = "This is a very long string which needs \
to wrap across multiple lines because \
otherwise my code is unreadable."
Both of the above methods result in identical strings.
Constructor
String()
-
Creates a new
String
object. It performs type conversion when called as a function, rather than as a constructor, which is usually more useful.
Static methods
String.fromCharCode(num1 [, ...[, numN]])
-
Returns a string created by using the specified sequence of Unicode values.
String.fromCodePoint(num1 [, ...[, numN)
-
Returns a string created by using the specified sequence of code points.
String.raw()
-
Returns a string created from a raw template string.
Instance properties
String.prototype.length
-
Reflects the
length
of the string. Read-only.
Instance methods
String.prototype.at(index)
-
Returns the character (exactly one UTF-16 code unit) at the specified
index
. Accepts negative integers, which count back from the last string character. String.prototype.charAt(index)
-
Returns the character (exactly one UTF-16 code unit) at the specified
index
. String.prototype.charCodeAt(index)
-
Returns a number that is the UTF-16 code unit value at the given
index
. String.prototype.codePointAt(pos)
-
Returns a nonnegative integer Number that is the code point value of the UTF-16 encoded code point starting at the specified
pos
. String.prototype.concat(str [, ...strN ])
-
Combines the text of two (or more) strings and returns a new string.
String.prototype.includes(searchString [, position])
-
Determines whether the calling string contains
searchString
. String.prototype.endsWith(searchString [, length])
-
Determines whether a string ends with the characters of the string
searchString
. String.prototype.indexOf(searchValue [, fromIndex])
-
Returns the index within the calling
String
object of the first occurrence ofsearchValue
, or-1
if not found. String.prototype.lastIndexOf(searchValue [, fromIndex])
-
Returns the index within the calling
String
object of the last occurrence ofsearchValue
, or-1
if not found. String.prototype.localeCompare(compareString [, locales [, options]])
-
Returns a number indicating whether the reference string
compareString
comes before, after, or is equivalent to the given string in sort order. String.prototype.match(regexp)
-
Used to match regular expression
regexp
against a string. String.prototype.matchAll(regexp)
-
Returns an iterator of all
regexp
's matches. String.prototype.normalize([form])
-
Returns the Unicode Normalization Form of the calling string value.
String.prototype.padEnd(targetLength [, padString])
-
Pads the current string from the end with a given string and returns a new string of the length
targetLength
. String.prototype.padStart(targetLength [, padString])
-
Pads the current string from the start with a given string and returns a new string of the length
targetLength
. String.prototype.repeat(count)
-
Returns a string consisting of the elements of the object repeated
count
times. String.prototype.replace(searchFor, replaceWith)
-
Used to replace occurrences of
searchFor
usingreplaceWith
.searchFor
may be a string or Regular Expression, andreplaceWith
may be a string or function. String.prototype.replaceAll(searchFor, replaceWith)
-
Used to replace all occurrences of
searchFor
usingreplaceWith
.searchFor
may be a string or Regular Expression, andreplaceWith
may be a string or function. String.prototype.search(regexp)
-
Search for a match between a regular expression
regexp
and the calling string. String.prototype.slice(beginIndex[, endIndex])
-
Extracts a section of a string and returns a new string.
String.prototype.split([sep [, limit] ])
-
Returns an array of strings populated by splitting the calling string at occurrences of the substring
sep
. String.prototype.startsWith(searchString [, length])
-
Determines whether the calling string begins with the characters of string
searchString
. String.prototype.substring(indexStart [, indexEnd])
-
Returns a new string containing characters of the calling string from (or between) the specified index (or indices).
String.prototype.toLocaleLowerCase( [locale, ...locales])
-
The characters within a string are converted to lowercase while respecting the current locale.
For most languages, this will return the same as
toLowerCase()
. String.prototype.toLocaleUpperCase( [locale, ...locales])
-
The characters within a string are converted to uppercase while respecting the current locale.
For most languages, this will return the same as
toUpperCase()
. String.prototype.toLowerCase()
-
Returns the calling string value converted to lowercase.
String.prototype.toString()
-
Returns a string representing the specified object. Overrides the
Object.prototype.toString()
method. String.prototype.toUpperCase()
-
Returns the calling string value converted to uppercase.
String.prototype.trim()
-
Trims whitespace from the beginning and end of the string. Part of the ECMAScript 5 standard.
String.prototype.trimStart()
-
Trims whitespace from the beginning of the string.
String.prototype.trimEnd()
-
Trims whitespace from the end of the string.
String.prototype.valueOf()
-
Returns the primitive value of the specified object. Overrides the
Object.prototype.valueOf()
method. String.prototype.@@iterator()
-
Returns a new iterator object that iterates over the code points of a String value, returning each code point as a String value.
HTML wrapper methods
Warning: Deprecated. Avoid these methods.
They are of limited use, as they provide only a subset of the available HTML tags and attributes.
String.prototype.anchor()
-
<a name="name">
(hypertext target) String.prototype.big()
<big>
String.prototype.blink()
<blink>
String.prototype.bold()
<b>
String.prototype.fixed()
<tt>
String.prototype.fontcolor()
<font color="color">
String.prototype.fontsize()
<font size="size">
String.prototype.italics()
<i>
String.prototype.link()
-
<a href="url">
(link to URL) String.prototype.small()
<small>
String.prototype.strike()
<strike>
String.prototype.sub()
<sub>
String.prototype.sup()
<sup>
Examples
String conversion
It's possible to use String
as a more reliable
toString()
alternative, as it works when
used on null
and undefined
. For example:
var nullVar = null;
nullVar.toString(); // TypeError: nullVar is null
String(nullVar); // "null"
var undefinedVar;
undefinedVar.toString(); // TypeError: undefinedVar is undefined
String(undefinedVar); // "undefined"
Specifications
Specification |
---|
ECMAScript Language Specification # sec-string-objects |
Browser compatibility
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