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 தமிழ்"

Since EcmaScript 6 String literals can also be so-called Template Strings:

`hello world`
`hello!
 world!`
`hello ${who}`
escape `<a>${who}</a>`

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

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:

var s1 = '2 + 2';             // creates a string primitive
var 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

Properties

String.prototype

Allows the addition of properties to a String object.

Methods

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.

String.raw()

Returns a string created from a raw template string.

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;
console.log(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

Methods

Methods unrelated to HTML

HTML wrapper methods

Examples

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

Specifications

Browser compatibility

See also

• DOMString
• StringView — a C-like representation of strings based on typed arrays
• Binary strings