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Chapter 10: Automatic Semicolon Insertion -

ASI

Examples

Rules of Automatic Semicolon Insertion

There are three basic rules of semicolon insertion:

1. When, as the program is parsed from left to right, a token (called the offending
token) is encountered that is not allowed by any production of the grammar, then
a semicolon is automatically inserted before the offending token if one or more of
the following conditions is true:

• The offending token is separated from the previous token by at least one
LineTerminator.
• The offending token is }.

2. When, as the program is parsed from left to right, the end of the input stream of
tokens is encountered and the parser is unable to parse the input token stream
as a single complete ECMAScript Program, then a semicolon is automatically
inserted at the end of the input stream.
3. When, as the program is parsed from left to right, a token is encountered that is
allowed by some production of the grammar, but the production is a restricted
production and the token would be the first token for a terminal or nonterminal
immediately following the annotation " [no LineTerminator here] " within the restricted
production (and therefore such a token is called a restricted token), and the
restricted token is separated from the previous token by at least one
LineTerminator, then a semicolon is automatically inserted before the restricted
token.

However, there is an additional overriding condition on the preceding rules: a
semicolon is never inserted automatically if the semicolon would then be parsed as an
empty statement or if that semicolon would become one of the two semicolons in the
header of a for statement (see 12.6.3).

Source: ECMA-262, Fifth Edition ECMAScript Specification:

Statements affected by automatic semicolon insertion

• empty statement
• var statement
• expression statement
• do-while statement
• continue statement

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