How Does JavaScript's Automatic Semicolon Insertion (ASI) Work and When Does It Occur?

Automatic Semicolon Insertion in JavaScript: Detailed Rules

JavaScript's automatic semicolon insertion (ASI) is a controversial feature that can insert semicolons at certain points in code where they are not explicitly written. Understanding ASI's rules is crucial to avoid potential bugs and ensure proper code execution.

ASI-Affected Statements

ASI only applies to specific types of statements:

• Empty statements
• Var statements
• Expression statements
• Do-while statements
• Continue statements
• Break statements
• Return statements
• Throw statements

ASI Rules

ECMAScript §11.9.1 outlines three cases where ASI can occur:

Case 1: Offending Token

If an invalid token is encountered and it follows a LineTerminator, a semicolon is inserted before it, except for '}'.

Case 2: End of Input Stream

If the parser cannot complete the program, a semicolon is inserted at the end of the input stream.

Case 3: Restricted Production

A semicolon is inserted before restricted tokens, which include:

• Update expressions without LineTerminators after operands (e.g., c)
• Continue and break statements without labels or semicolons
• Return statements without expressions
• Throw statements without expressions
• Arrow functions without parentheses after parameters
• Yield expressions without parentheses after * or yield

Example of ASI in Practice

Consider the following code:

return
"something";

According to Case 3, ASI inserts a semicolon before the restricted production return statement, resulting in:

return;
"something";

Conclusion

ASI can be a tricky feature to handle, but understanding its rules is essential for writing robust JavaScript code. By adhering to these rules, developers can avoid errors and ensure the correct execution of their programs.

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