The Sneaky JavaScript Bug That Hides in Plain Sight — A 0 That Broke My Code

A common JavaScript bug occurs when checking for values that could legitimately be zero. In a twoSum function finding indices of numbers adding to a target, using if(map[complement]) fails when an index is 0, as JavaScript treats 0 as falsy. The code works for most cases but silently fails when a valid match includes the first array element. The solution is to use hasOwnProperty() or other existence-checking methods. This bug can appear in real applications like form validation, where elements with ID 0 are incorrectly skipped, highlighting the importance of careful truthiness evaluations in JavaScript.