JavaScript: The Definitive Guide, 6th Edition

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A string is an immutable ordered sequence of 16-bit values, each of which typically represents a Unicode character—strings are JavaScript’s type for representing text. The length of a string is the number of 16-bit values it contains. JavaScript’s strings (and its arrays) use zero-based indexing: the first 16-bit value is at position 0, the second at position 1 and so on. The empty string is the string of length 0. JavaScript does not have a special type that represents a single element of a string. To represent a single 16-bit value, simply use a string that has a length of 1.

Characters, Codepoints, and JavaScript Strings

JavaScript uses the UTF-16 encoding of the Unicode character set, and JavaScript strings are sequences of unsigned 16-bit values. The most commonly used Unicode characters (those from the “basic multilingual plane”) have codepoints that fit in 16 bits and can be represented by a single element of a string. Unicode characters whose codepoints do not fit in 16 bits are encoded following the rules of UTF-16 as a sequence (known as a “surrogate pair”) of two 16-bit values. This means that a JavaScript string of length 2 (two 16-bit values) might represent only a single Unicode character:

var p = "π"; // π is 1 character with 16-bit codepoint 0x03c0
var e = "e"; // e is 1 character with 17-bit codepoint 0x1d452
p.length     // => 1: p consists of 1 16-bit element
e.length     // => 2: UTF-16 encoding of e is 2 16-bit values: "\ud835\udc52"

The various string-manipulation ...