String.fromCharCode() - JavaScript 编辑

The static String.fromCharCode() method returns a string created from the specified sequence of UTF-16 code units.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

String.fromCharCode(num1[, ...[, numN]])

Parameters

num1, ..., numN

A sequence of numbers that are UTF-16 code units. The range is between 0 and 65535 (0xFFFF). Numbers greater than 0xFFFF are truncated. No validity checks are performed.

Return value

A string of length N consisting of the N specified UTF-16 code units.

Description

This method returns a string and not a String object.

Because fromCharCode() is a static method of String, you always use it as String.fromCharCode(), rather than as a method of a String object you created.

Returning supplementary characters

In UTF-16, the most common characters can be represented by a single 16-bit value (i.e. a code unit). However, this set of characters, known as the Base Multilingual Plane (BMP), is only ¹/₁₇^th of the total addressable Unicode code points. The remaining code points, in the range of 65536 (0x010000) to 1114111 (0x10FFFF) are known as supplementary characters. In UTF-16, supplementary characters are represented by two 16-bit code units, known as surrogates, that were reserved for this purpose. A valid combination of two surrogates used to represent a supplementary character is known as a surrogate pair.

Because fromCharCode() only works with 16-bit values (same as the \u escape sequence), a surrogate pair is required in order to return a supplementary character. For example, both String.fromCharCode(0xD83C, 0xDF03) and \uD83C\uDF03 return code point U+1F303 "Night with Stars".

While there is a mathematical relationship between the supplementary code point value (e.g. 0x1F303) and both surrogate values that represent it (e.g., 0xD83C and 0xDF03), it does require an extra step to either calculate or look up the surrogate pair values every time a supplementary code point is to be used. For this reason, it's more convenient to use String.fromCodePoint() (part of the ES2015 standard), which allows for returning supplementary characters based on their actual code point value. For example, String.fromCodePoint(0x1F303) returns code point U+1F303 "Night with Stars".

Examples

Using fromCharCode()

BMP characters, in UTF-16, use a single code unit:

String.fromCharCode(65, 66, 67);   // returns "ABC"
String.fromCharCode(0x2014);       // returns "—"
String.fromCharCode(0x12014);      // also returns "—"; the digit 1 is truncated and ignored
String.fromCharCode(8212);         // also returns "—"; 8212 is the decimal form of 0x2014

Supplementary characters, in UTF-16, require two code units (i.e. a surrogate pair):

String.fromCharCode(0xD83C, 0xDF03); // Code Point U+1F303 "Night with
String.fromCharCode(55356, 57091);   // Stars" == "\uD83C\uDF03"

String.fromCharCode(0xD834, 0xDF06, 0x61, 0xD834, 0xDF07); // "\uD834\uDF06a\uD834\uDF07"

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'String.fromCharCode' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• String.fromCodePoint()
• String.prototype.charAt()
• String.prototype.charCodeAt()
• String.prototype.codePointAt()