num1, …, numN

A number between 0 and 65535 (0xFFFF) representing a UTF-16 code unit. Numbers greater than 0xFFFF are truncated to the last 16 bits. No validity checks are performed.

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

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

Unicode code points range from 0 to 1114111 (0x10FFFF). charCodeAt() always returns a value that is less than 65536, because the higher code points are represented by a pair of 16-bit surrogate pseudo-characters. Therefore, in order to produce a full character with value greater than 65535, it is necessary to provide two code units (as if manipulating a string with two characters). For information on Unicode, see UTF-16 characters, Unicode code points, and grapheme clusters.

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(), 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".

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"

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