String

Table of contents

  1. 1. Summary
  2. 2. Syntax
  3. 3. Parameters
  4. 4. Description
    1. 4.1. Character access
    2. 4.2. Comparing strings
  5. 5. Properties
  6. 6. Methods
  7. 7. String instances
    1. 7.1. Properties
    2. 7.2. Methods
      1. 7.2.1. Methods unrelated to HTML
      2. 7.2.2. HTML wrapper methods
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Summary

String is a global object that may be used to construct String instances.

Syntax

String([string])
new String([string])

String literals take the form:

'stringText'
"stringText"

Parameters

string
Any string.
stringText
Any series of characters that has been properly encoded.

Description

String objects may be created by calling the constructor new String(). The String object wraps JavaScript's string primitive data type with the methods described below. The global function String() can also be called without new in front to create a primitive string. String literals in JavaScript are primitive strings.

Because JavaScript automatically converts between string primitives and String objects, you can call any of the methods of the String object on a string primitive. JavaScript automatically converts the string primitive to a temporary String object, calls the method, then discards the temporary String object. For example, you can use the String.length property on a string primitive created from a string literal:

s_obj = new String(s_prim = s_also_prim = "foo");

s_obj.length;       // 3
s_prim.length;      // 3
s_also_prim.length; // 3
'foo'.length;       // 3
"foo".length;       // 3

(A string literal is denoted with single or double quotation marks.)

String objects can be converted to primitive strings with the valueOf method.

String primitives and String objects give different results when evaluated as JavaScript. Primitives are treated as source code; String objects are treated as a character sequence object. For example:

s1 = "2 + 2";               // creates a string primitive
s2 = new String("2 + 2");   // creates a String object
eval(s1);                   // returns the number 4
eval(s2);                   // returns the string "2 + 2"
eval(s2.valueOf());         // returns the number 4

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 is to treat the string as an array, where each index corresponds to an individual character:

return 'cat'[1]; // returns "a"
The second way (treating the string as an array) is not part of ECMAScript 3; it's a JavaScript and ECMAScript 5 feature (and not supported in all browsers).

In both cases, attempting to set an individual character won't work. Trying to set a character through charAt results in an error, while trying to set a character via indexing does not throw an error, but the string itself is unchanged.

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.

Properties

For properties inherited by String instances, see Properties of String instances.

prototype
Allows the addition of properties to a String object.
Properties inherited from Function:
arity, caller, constructor, length, name

Methods

For methods inherited by String instances, see Methods of String instances.

fromCharCode
Returns a string created by using the specified sequence of Unicode values.
Methods inherited from Function:
apply, call, toSource, toString

String instances

Properties

constructor
Specifies the function that creates an object's prototype.
length
Reflects the length of the string.
N
Non-standard
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:
__parent__, __proto__

Methods

Methods unrelated to HTML

charAt
Returns the character at the specified index.
charCodeAt
Returns a number indicating the Unicode value of the character at the given index.
concat
Combines the text of two strings and returns a new string.
indexOf
Returns the index within the calling String object of the first occurrence of the specified value, or -1 if not found.
lastIndexOf
Returns the index within the calling String object of the last occurrence of the specified value, or -1 if not found.
localeCompare
Returns a number indicating whether a reference string comes before or after or is the same as the given string in sort order.
match
Used to match a regular expression against a string.
quote
Non-standard
Wraps the string in double quotes (""").
replace
Used to find a match between a regular expression and a string, and to replace the matched substring with a new substring.
search
Executes the search for a match between a regular expression and a specified string.
slice
Extracts a section of a string and returns a new string.
split
Splits a String object into an array of strings by separating the string into substrings.
substr
Returns the characters in a string beginning at the specified location through the specified number of characters.
substring
Returns the characters in a string between two indexes into the string.
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.
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.
toLowerCase
Returns the calling string value converted to lower case.
toSource
Non-standard
Returns an object literal representing the specified object; you can use this value to create a new object. Overrides the Object.toSource method.
toString
Returns a string representing the specified object. Overrides the Object.toString method.
toUpperCase
Returns the calling string value converted to uppercase.
trim
New in Firefox 3.5
Trims whitespace from the beginning and end of the string. Part of the ECMAScript 5 standard.
trimLeft
New in Firefox 3.5 Non-standard
Trims whitespace from the left side of the string.
trimRight
New in Firefox 3.5 Non-standard
Trims whitespace from the right side of the string.
valueOf
Returns the primitive value of the specified object. Overrides the Object.valueOf method.

HTML wrapper methods

Non-standard

Each of the following methods returns a copy of the string wrapped inside the appropriate HTML tag.

anchor
<a name="name"> (hypertext target)
big
<big>
blink
<blink>
bold
<b>
fixed
<tt>
fontcolor
<font color="color">
fontsize
<font size="size">
italics
<i>
link
<a href=/old?u=https%3A%2F%2Fdeveloper.mozilla.org%2Fen%2FJavaScript%2FReference%2FGlobal_Objects%2F%26quot%3B%3Cem&y=1999>url"> (link to URL)
small
<small>.
strike
<strike>
sub
<sub>
sup
<sup>

These methods are of limited use, as they provide only a subset of the available HTML tags and attributes.

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