ECMAScript source text is represented as a sequence of characters in the Unicode character encoding, version 3.0 or later. The text is expected to have been normalised to Unicode Normalization Form C (canonical composition), as described in Unicode Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as though they were performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of 16-bit code units for the purposes of this specification. Such a source text may include sequences of 16-bit code units that are not valid UTF-16 character encodings. If an actual source text is encoded in a form other than 16-bit code units it must be processed as if it was first converted to UTF-16.
Throughout the rest of this document, the phrase “code unit” and the word “character” will be used to refer to a 16-bit unsigned value used to represent a single 16-bit unit of text. The phrase “Unicode character” will be used to refer to the abstract linguistic or typographical unit represented by a single Unicode scalar value (which may be longer than 16 bits and thus may be represented by more than one code unit). The phrase “code point” refers to such a Unicode scalar value. “Unicode character” only refers to entities represented by single Unicode scalar values: the components of a combining character sequence are still individual “Unicode characters,” even though a user might think of the whole sequence as a single character.
In string literals, regular expression literals, and identifiers, any character (code unit) may also be expressed as a Unicode escape sequence consisting of six characters, namely
\u plus four hexadecimal digits. Within a comment, such an escape sequence is effectively ignored as part of the comment. Within a string literal or regular expression literal, the Unicode escape sequence contributes one character to the value of the literal. Within an identifier, the escape sequence contributes one character to the identifier.
NOTE Although this document sometimes refers to a “transformation” between a “character” within a “string” and the 16-bit unsigned integer that is the code unit of that character, there is actually no transformation because a “character” within a “string” is actually represented using that 16-bit unsigned value.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the
Unicode escape sequence
\u000A, for example, occurs within a single-line comment, it is interpreted as a line
terminator (Unicode character
000A is line feed) and therefore the next character is not part of the comment.
Similarly, if the Unicode escape sequence
\u000A occurs within a string literal in a Java program, it is likewise
interpreted as a line terminator, which is not allowed within a string literal—one must write
\n instead of
\u000A to cause a line feed to be part of the string value of a string literal. In an ECMAScript program, a Unicode
escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment.
Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes a character
to the String value of the literal and is never interpreted as a line terminator or as a quote mark that might terminate the