String

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概要 - Summary

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

String 全局对象是一个字符串的构造函数,或者说是一个字符的序列。

语法 - Syntax

String literals take the forms:

字符串字面量的形式如下:

'string text'
"string text"

Or, using the String global object directly:

或者,可以直接使用 String 全局对象:

String(thing)
new String(thing)

参数 - Parameters

thing
Anything to be converted to a string.
任何可以被转换为字符串的东西。

描述 - 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, and checking for the existence or location of substrings with the substring and substr methods.

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-like object, where individual characters correspond to a numerical index:

return 'cat'[1]; // returns "a"
Array-like character access (the second way above) is not part of ECMAScript 3. It is a JavaScript and ECMAScript 5 feature.

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 booleans 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 and 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.

String object can always be converted to its primitive counterpart with the valueOf method.

console.log(eval(s2.valueOf())); // returns the number 4

Properties

{{ Js_see_prototype("String", "Properties") }}

prototype
Allows the addition of properties to a String object.
{{ jsOverrides("Function", "Properties", ["prototype"]) }}

Methods

{{ Js_see_prototype("String", "Methods") }}

fromCharCode
Returns a string created by using the specified sequence of Unicode values.
{{ jsOverrides("Function", "Methods", ["fromCharCode"]) }}

String instances

Properties

{{ page('en/JavaScript/Reference/Global_Objects/String/prototype', 'Properties') }}

Methods

{{ page('en/JavaScript/Reference/Global_Objects/String/prototype', 'Methods') }}

Example

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]));
}

{{ languages( { "es": "es/Referencia_de_JavaScript_1.5/Objetos_globales/String", "fr": "fr/R\u00e9f\u00e9rence_de_JavaScript_1.5_Core/Objets_globaux/String", "ja": "ja/Core_JavaScript_1.5_Reference/Global_Objects/String", "pl": "pl/Dokumentacja_j\u0119zyka_JavaScript_1.5/Obiekty/String" } ) }}

Browser compatibility

{{ CompatibilityTable() }}

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 0.2 {{ CompatVersionUnknown() }} 9.0 {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }}
Feature Android Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }}

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