How Does JavaScript's Automatic Semicolon Insertion (ASI) Work: Rules and Exceptions?

ASI in JavaScript: Rules and Exceptions

Automatic semicolon insertion (ASI) is a feature of JavaScript that automatically adds semicolons at specific points in the code. Understanding the rules for ASI is crucial to prevent potential bugs.

Statements Affected by ASI

The following statements are affected by ASI:

• Empty statements
• Variable declarations (var)
• Expression statements
• Control flow statements (do-while, continue, break)
• Return statements
• Throw statements

ASI Rules

ASI is applied in three main cases:

1. Invalid Token: When an unexpected token is encountered, a semicolon is inserted before it if:

   • The token is preceded by a line break.
   • The token is the closing curly brace }.
2. End of Input Stream: If the end of the input stream is reached without a complete Program, a semicolon is inserted at the end.

3. Restricted Tokens: Semicolons are automatically inserted before tokens that follow restricted productions in the grammar. This includes tokens without line terminators:

   • Update expressions ( , --)
   • Control flow statements (continue, break)
   • Return statements
   • Throw statements
   • Generator function expressions
   • Yield expressions

Example 1 (Invalid Token):

{ 1
2 } 3

ASI transforms this code into:

{ 1
;2 ;} 3;

Example 2 (End of Input Stream):

a = b
++c

ASI adds a semicolon at the end:

a = b;
++c;

Example 3 (Restricted Token):

return
  "something";

ASI inserts a semicolon before the restricted return token:

return;
  "something";

Note: While ASI can provide convenience, it's important to be aware of its potential impact and to use semicolons explicitly for clarity and consistency.

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