Does JavaScript's Automatic Semicolon Insertion Always Prevent Errors?

The Nuances of JavaScript's Automatic Semicolon Insertion

Clarifying ASI's Applicability

Automatic semicolon insertion (ASI) is a JavaScript feature that automatically inserts a semicolon into the code when it is missing and would not cause syntactic errors. However, it only applies to specific statement types:

• Empty statements
• Variable declarations
• Expression statements
• Do-while statements
• Continue statements
• Break statements
• Return statements
• Throw statements

ASI Rules as per ECMAScript Specification

The ECMAScript specification defines three scenarios for ASI:

1. Invalid Token Encountered

An offending token not permitted by the grammar triggers semicolon insertion if:

• It is separated from the previous token by a line break.
• It is followed by }.

2. End of Input Stream

If the input stream ends without the parser being able to parse a complete program, a semicolon is inserted at the end.

3. Restricted Productions

When a token is allowed but belongs to a "restricted production" (e.g., return or continue without a line break), a semicolon is inserted before it.

Example:

return
"something";

is transformed to:

return;
"something";

Exception to the Rule

However, there is an exception to the rules. If the ASI would result in a SyntaxError, it is not inserted. For instance, in the following code:

if (x)
return
  y;

ASI would transform it to:

if (x)
return;
y;

This would cause a SyntaxError because y is not a valid statement on its own.

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