1363325 - number of steps at for-loop depends on whitespace

Status: RESOLVED FIXED

(DevTools :: Debugger, defect)

Description

[Daniel Lehmann]

Attachment: example.html

User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:53.0) Gecko/20100101 Firefox/53.0
Build ID: 20170418123106

Steps to reproduce:

1. Open attached example.html in Firefox 53.0 or Firefox Developer Edition 54.0a2 (both affected).
2. Open Developer Tools, go to Debugger.
3. Set breakpoints in lines 8, 12, 19, 27, and 37 (all the for loops).
4. Reload to hit first breakpoint.
5. Click "Step In" until past the for-loop.
6. Click "Resume" to hit next for-loop breakpoint.
7. Repeat 5. and 6. until through all loops.


Actual results:

- Depending on the whitespace, one needs to click step in 1, 7, 10, 11, or 12 times to get through a 3-iteration for-loop.
- In particular line 8 in example: For-loop on single line can only be "stepped" once until reaching next statement past the loop.
- In particular line 37 in example: Debugger allows to set breakpoint and pauses at "for (" line (without any initialization, condition, or increment substatement).


Expected results:

- Most confusing: single-line loop, line 8 in example: At least 3 steps should be needed for this 3-iteration loop. Otherwise, developers could think the loop did not execute all iterations.
- "for (" line without any substatement: Setting breakpoint at line 37 doesn't make sense, since there is no statement in this line. Debugger should slide breakpoint to loop-variable initialization statement in next line. This way, the third testcase in the example (line 27) and this (line 37) would need the same number of steps and be consistent.
- (Subjective:) In general, number of steps needed to get past a statement should not depend on its formatting. The debugger should pause at each substatement, even if they are on the same line. For for-loops this means potentially pausing three times at the loop header line, once for initialization ("let i = 0"), once for condition statement ("i < 3"), once for increment ("i++").

Note: Chrome needs the same number of steps for all 4 cases. It pauses at initialization, at the condition, at the increment, and the statement in the loop body.

Comment 1

[Daniel Lehmann]

Attachment: Video capture of bug in Firefox Developer Edition 54.0a2

Comment 2

[Jason Laster [:jlast]]

I believe this has been fixed recently.

Daniel, could you try it again?

Comment 3

[Daniel Lehmann]

Hi Jason, sorry for the late reply.
Yes, I cannot reproduce the bug any longer with 66.0.5 (64-bit) on Ubuntu. So it seems fixed! Thanks a lot and keep up the great work :)

Comment 4

[Logan Smyth [:loganfsmyth]]

Thanks for verifying!