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Does JavaScript's Automatic Semicolon Insertion Always Prevent Errors?

Susan Sarandon
Release: 2024-12-23 16:05:15
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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";
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is transformed to:

return;
"something";
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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;
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ASI would transform it to:

if (x)
return;
y;
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This would cause a SyntaxError because y is not a valid statement on its own.

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