String

Summary

The String global object is a constructor for strings, or a sequence of characters.

Syntax

String literals take the forms:

'string text'
"string text"
"中文 español English हिन्दी العربية português বাংলা русский 日本語 ਪੰਜਾਬੀ 한국어"

Beside regular, printable characters, special characters can be encoded using escape notation:

Code Output
\' single quote
\" double quote
\\ backslash
\n new line
\r carriage return
\t tab
\b backspace
\f form feed
\u.... unicode codepoint

Or, using the String global object directly:

String(thing)
new String(thing)

Parameters

thing
Anything to be converted to a string.

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.

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"

For character access using bracket notation, 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

C developers have the strcmp() function for comparing strings. In JavaScript, you just use the less-than and greater-than operators:

var a = "a";
var b = "b";
if (a < b) // true
  print(a + " is less than " + b);
else if (a > b)
  print(a + " is greater than " + b);
else
  print(a + " and " + b + " are equal.");

A similar result can be achieved using the localeCompare method inherited by String instances.

Distinction between 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 (i.e., 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.

var s_prim = "foo";
var 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:

s1 = "2 + 2";               // creates a string primitive
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, 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
Note: For another possible approach to strings in JavaScript, please read the article about StringView – a C-like representation of strings based on typed arrays.

Properties

For properties available on String instances, see Properties of String instances.

String.prototype
Allows the addition of properties to a String object.
Properties inherited from Function:

Methods

For methods available on String instances, see Methods of String instances.

String.fromCharCode()
Returns a string created by using the specified sequence of Unicode values.
String.fromCodePoint()
Returns a string created by using the specified sequence of code points.
Methods inherited from Function:

String generic methods

The String instance methods are also available in Firefox as of JavaScript 1.6 (though not part of the ECMAScript standard) on the String object for applying String methods to any object:

var num = 15;
alert(String.replace(num, /5/, '2'));

Generics are also available on Array methods.

The following is a shim to provide support to non-supporting browsers:

/*globals define*/
// Assumes all supplied String instance methods already present
// (one may use shims for these if not available)
(function () {
    'use strict';

    var i,
        // We could also build the array of methods with the following, but the
        //   getOwnPropertyNames() method is non-shimable:
        // Object.getOwnPropertyNames(String).filter(function (methodName)
        //  {return typeof String[methodName] === 'function'});
        methods = [
            'quote', 'substring', 'toLowerCase', 'toUpperCase', 'charAt',
            'charCodeAt', 'indexOf', 'lastIndexOf', 'startsWith', 'endsWith',
            'trim', 'trimLeft', 'trimRight', 'toLocaleLowerCase',
            'toLocaleUpperCase', 'localeCompare', 'match', 'search',
            'replace', 'split', 'substr', 'concat', 'slice'
        ],
        methodCount = methods.length,
        assignStringGeneric = function (methodName) {
            var method = String.prototype[methodName];
            String[methodName] = function (arg1) {
                return method.apply(arg1, Array.prototype.slice.call(arguments, 1));
            };
        };

    for (i = 0; i < methodCount; i++) {
        assignStringGeneric(methods[i]);
    }
}());

String instances

Properties

String.prototype.constructor
Specifies the function that creates an object's prototype.
String.prototype.length
Reflects the length of the string.
N
Used to access the character in the Nth position where N is a positive integer between 0 and one less than the value of length. These properties are read-only.
Properties inherited from Object:

Methods unrelated to HTML

String.prototype.charAt()
Returns the character at the specified index.
String.prototype.charCodeAt()
Returns a number indicating the Unicode value of the character at the given index.
String.prototype.codePointAt()
Returns a non-negative integer that is the UTF-16 encoded code point value at the given position.
String.prototype.concat()
Combines the text of two strings and returns a new string.
String.prototype.contains()
Determines whether one string may be found within another string.
String.prototype.endsWith()
Determines whether a string ends with the characters of another string.
String.prototype.indexOf()
Returns the index within the calling String object of the first occurrence of the specified value, or -1 if not found.
String.prototype.lastIndexOf()
Returns the index within the calling String object of the last occurrence of the specified value, or -1 if not found.
String.prototype.localeCompare()
Returns a number indicating whether a reference string comes before or after or is the same as the given string in sort order.
String.prototype.match()
Used to match a regular expression against a string.
String.prototype.normalize()
Returns the Unicode Normalization Form of the calling string value.
String.prototype.quote()
Wraps the string in double quotes (""").
String.prototype.repeat()
Returns a string consisting of the elements of the object repeated the given times.
String.prototype.replace()
Used to find a match between a regular expression and a string, and to replace the matched substring with a new substring.
String.prototype.search()
Executes the search for a match between a regular expression and a specified string.
String.prototype.slice()
Extracts a section of a string and returns a new string.
String.prototype.split()
Splits a String object into an array of strings by separating the string into substrings.
String.prototype.startsWith()
Determines whether a string begins with the characters of another string.
String.prototype.substr()
Returns the characters in a string beginning at the specified location through the specified number of characters.
String.prototype.substring()
Returns the characters in a string between two indexes into the string.
String.prototype.toLocaleLowerCase()
The characters within a string are converted to lower case while respecting the current locale. For most languages, this will return the same as toLowerCase.
String.prototype.toLocaleUpperCase()
The characters within a string are converted to upper case 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 lower case.
String.prototype.toSource()
Returns an object literal representing the specified object; you can use this value to create a new object. Overrides the Object.prototype.toSource method.
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.trimLeft()
Trims whitespace from the left side of the string.
String.prototype.trimRight()
Trims whitespace from the right side of the string.
String.prototype.valueOf()
Returns the primitive value of the specified object. Overrides the Object.prototype.valueOf method.

HTML wrapper methods

These methods 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 "safer" toString alternative, as although it still normally calls the underlying toString, it also works for null and undefined. For example:

var outputStrings = [];
for (let i = 0, n = inputValues.length; i < n; ++i) {
  outputStrings.push(String(inputValues[i]));
}

Specifications

Specification Status Comment
ECMAScript 1st Edition. Standard Initial definition.
ECMAScript Language Specification 5.1th Edition (ECMA-262) Standard  
ECMAScript Language Specification 6th Edition (ECMA-262) Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 0.2 (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

See also

Hide Sidebar