What are the rules for JavaScript’s automatic semicolon insertion (ASI)?

First of all you should know which statements are affected by the automatic semicolon insertion (also known as ASI for brevity):

• empty statement
• var statement
• expression statement
• do-while statement
• continue statement
• break statement
• return statement
• throw statement

The concrete rules of ASI, are described in the specification §11.9.1 Rules of Automatic Semicolon Insertion

Three cases are described:

1. When an offending token is encountered that is not allowed by the grammar, a semicolon is inserted before it if:

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

e.g.:

    { 1
    2 } 3

is transformed to

    { 1
    ;2 ;} 3;

The NumericLiteral 1 meets the first condition, the following token is a line terminator.
The 2 meets the second condition, the following token is }.

2. When 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 Program, then a semicolon is automatically inserted at the end of the input stream.

e.g.:

    a = b
    ++c

is transformed to:

    a = b;
    ++c;

3. This case occurs when a token is allowed by some production of the grammar, but the production is a restricted production, a semicolon is automatically inserted before the restricted token.

Restricted productions:

    UpdateExpression :
        LeftHandSideExpression [no LineTerminator here] ++
        LeftHandSideExpression [no LineTerminator here] --
    
    ContinueStatement :
        continue ;
        continue [no LineTerminator here] LabelIdentifier ;
    
    BreakStatement :
        break ;
        break [no LineTerminator here] LabelIdentifier ;
    
    ReturnStatement :
        return ;
        return [no LineTerminator here] Expression ;
    
    ThrowStatement :
        throw [no LineTerminator here] Expression ; 

    ArrowFunction :
        ArrowParameters [no LineTerminator here] => ConciseBody

    YieldExpression :
        yield [no LineTerminator here] * AssignmentExpression
        yield [no LineTerminator here] AssignmentExpression

The classic example, with the ReturnStatement:

    return 
      "something";

is transformed to

    return;
      "something";