Checklist for debugging the issue

Listed in rough order of importance

Simplify the problem relentlessly

Comment out all unrelated code until the bug goes away
Reduce the number of moving parts (hardcode variables, etc)
Disable libraries

Understand the problem

Get to the root cause by asking "why" repeatedly
Understand when the problem occurs and when it doesn't occur
Search the codebase for all occurences of unique keywords to see all related areas of the code

Search for related problems and solutions

Your codebase
Google
Stack Overflow
GitHub Issues
Past experience

Exaggerate the problem and make it more obvious

Ex: If you are writing a method to shrink an image to fit in a bounding box, and the ratios aren’t working properly, exaggerate the ratios. Pick ratios like 100 by 5.
Ex: If there is a layout issue on your HTML page, add a red outline to all the elements.
Ex: If you suspect a timing issue, add an intentional delay to check your assumption.

Tighten the feedback loop

Conditional breakpoints so you don't have to hit next so many times
Run an individual test instead of all of them
Use the javascript console instead of modifying the code

Guess and test

If you suspect what the problem is, ask "what is the simplest way to test my theory?"
- Add a debugger statement or conditional debugger statement
- Raise an exception to see if a line is hit
- Hardcode values