Request: add option for no-fallthrough with newlines

Environment

Node version: v16.14.0 npm version: v8.3.1 Local ESLint version: v8.11.0 (Currently used) Global ESLint version: v7.23.0 Operating System: darwin 21.1.0

What parser are you using?

Default (Espree)

What did you do?

var x = 3;
switch(x){
  case 0:
  case 1: break;

  case 2:
    
  case 3:
    break;
}

ESLint Demo Link

What did you expect to happen?

No errors

What actually happened?

  8:3  error  Expected a 'break' statement before 'case'  no-fallthrough

Participation

• I am willing to submit a pull request for this issue.

Additional comments

It might also make sense to support comments:

switch(x) {
  // currently accepted
  case 0:
  case 1:
    break;

  // this issue
  case 2:

  case 3:
    break;

  // general proposal
  /* a note about 4 */
  case 4:
  /* a note about 5 */
  case 5:
    break;
}

[reviewher]

Coming from other programming languages, ESLint’s current definition of fallthrough is inconsistent. @SheetJSDev talked about C/C++. Other languages:

PHP: PSR2 guidelines define fallthrough as requiring at least one statement. This is the same as C/C++

Java: Sun style defines a fallthrough as “doesn’t include a break statement”, so technically you always need a fallthrough comment between cases, no matter how much whitespace is in between.

Go: forced breaks at the end of each case clause, even if there is no whitespace. Special keyword fallthrough to fall through.

Perl: given / when statements do not fall through. Special keyword continue to fall through.

C#: compilation error if there is no break between case clauses, irrespective of the number of lines. Special statement goto case ____ to fall through

Ruby: case / when statements do not fall through

Rust: match does not have fallthrough support.

Pascal: case statement has no fallthrough

In summary, “everything but eslint” defines fallthrough in one of two ways:

“intrinsic”: each case is separate. case a: case b: are seen as two distinct parts irrespective of the amount of whitespace and newlines between them.

“statement”: at least one statement between case clauses with no final break.

ESLint’s behavior is surprising and feels inconsistent.

[reviewher]

I genuinely tried to find an example outside of ESLint and came up short. Even in JS land:

JSHint takes the “statement” interpretation (you can verify on https://jshint.com/ )

JSLint takes the “statement” interpretation (you can verify on https://www.jslint.com/ )

ESLint “blazing a new trail” is not inherently bad, but there arguably should be a very compelling reason.