JavaScript Array and Object Methods Cheat Sheet

Part 16 of 16 · ~2 min

Common Mistakes Worth Remembering

• Treating .sort() and .reverse() as if they were as harmless as .map() or .filter() — the first two rewrite your array right where it sits; the second two never touch it.
• Sorting a list of numbers with a bare .sort() and being surprised the result isn't in numeric order — it was never going to be, without a comparator telling it what to compare on.
• Removing a candidate with delete candidates[i] instead of .splice(), which leaves a hole rather than actually shrinking the array — .length stays the same, and .forEach()/.map() quietly skip straight over the gap.
• Reaching for .forEach() when you actually need something back, or trying to stop it partway through with break — it returns nothing and can't be interrupted; .map(), .reduce(), or a plain for...of loop can do either.
• Skipping the initial value on a .reduce() call whose array could plausibly be empty someday — harmless for months, then a TypeError on the first day it actually is.
• Reaching for || to supply a fallback when 0, "", or false might be a genuine, intentional value rather than a missing one — ?? is almost always what belongs there instead.
• Treating spread or Object.assign() as if they cloned everything — they only duplicate the outer layer, so editing a nested field afterward changes the "original" too, not just your supposed copy.
• Expecting === to tell you whether two objects hold the same data — it only ever answers whether they're the same object, which is a different question.
• Reaching for a plain object on a problem that's genuinely Map-shaped — non-string keys, constant adding and removing, actually needing to trust .size — and then losing an afternoon to why a numeric-looking key won't match.

If any of this felt familiar from a different angle, that's not a coincidence — useState and useReducer, covered in React Fundamentals, lean on this exact same set of methods, and the reason React insists on a brand-new array or object every time state changes traces straight back to the very first section above. The scope rules, closures, and event-loop timing that all of this code actually runs inside of get their own full treatment in JavaScript Core Concepts.

A reasonable next step: open the code lab and run a few of these against a dataset you put together yourself, not just the candidates above.