Common bug patterns in Python and JavaScript
Most bugs are not new. They are the same handful of shapes, over and over. Learn to recognise the shape and you find the next one in minutes instead of hours.
Off-by-one errors
The loop that runs one time too many, the slice that drops the last item, the index that starts in the wrong place.
IndexError: list index out of range
Infinite loops
The tab freezes, nothing prints, and the loop condition never becomes false.
while loop never ends
Null and undefined errors
Cannot read properties of undefined, NoneType has no attribute, NaN appearing from nowhere.
TypeError: Cannot read properties of undefined (reading 'x')
Type errors
Totals that concatenate, sorts that put 10 before 9, comparisons that refuse to run.
TypeError: unsupported operand type(s)
Scope and closure bugs
Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.
UnboundLocalError: cannot access local variable
Async and race conditions
A missing await, results in the wrong order, and updates that overwrite each other under load.
[object Promise] in output
Logic errors
The wrong operator, an inverted condition, a formula the right way round but backwards.
wrong output but no error
Mutation and copying bugs
Assignment does not copy, a shallow copy still shares its contents, and mutating methods return None.
editing copy changes original
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