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

Question

Well, first I should probably ask if this is browser dependent.

I've read that if an invalid token is found, but the section of code is valid until that invalid token, a semicolon is inserted before the token if it is preceded by a line break.

However, the common example cited for bugs caused by semicolon insertion is:

return
  _a+b;

..which doesn't seem to follow this rule, since _a would be a valid token.

On the other hand, breaking up call chains works as expected:

$('#myButton')
  .click(function(){alert("Hello!")});

Does anyone have a more in-depth description of the rules?

Answer 1

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";