When Is It Dangerous To Line Break After A Close Parenthesis In Javascript?

What are the dangers of breaking a line of long code up at a close parenthesis? When could a semicolon be automatically inserted by Javascript (presumably that is the danger, righ

Solution 1:

When you're trying to return an object,

return {
 'foo': 'bar'
}

will return the object, whereas

return
{
  'foo': 'bar'
}

will return undefined. Javascript will automatically insert a semicolon after return in the second example, and the object will never be reached.

For functions, because function() isn't valid on its own, it shouldn't make a difference if the brace is on the same line or the next.

See also section 7.9.1 Rules of Automatic Semicolon Insertion of the ECMAScript specification. Aside from return, there are four other situations where a semicolon will be inserted due to a newline: break, continue, throw and ++ or --.

When a continue, break, return, or throw token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, or throw token.

Solution 2:

A semicolon will be inserted only if the following line is not a valid continuation of the previous line (see exceptions below). So function() { with { on the next line is always safe.

From the ECMAScript spec:

When, as the program is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:

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

• The offending token is }.

Exceptions to this rule are the increment/decrement operators, continue, break, return and throw, whose arguments must always be on the same line:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.

When a continue, break, return, or throw token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, or throw token.

Solution 3:

It is safe.

I took a quick look at the semicolon insertion rules in the spec (3rd edition), and for function declarations it is OK.