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The Wiped Field

Someone changed an order status and the delivery note vanished. The client only sent status. Work out the difference between "not sent" and "sent as empty".

Beginner15 min on the clock5 graded checks · 100 pointsThe Write Side

Graded by running your code against real cases.

What you build

applyPatch(row, patch) — a field that was not sent is left alone
A field sent as null IS cleared. Those are different instructions
replace(row, next) — PUT semantics: what you did not send is gone
patchRow(rows, id, patch) — update one row in a list
changedFields(row, patch) — what would actually change

Done means

A patch of one field changes exactly one field, and null still clears.

How it is graded

Published in full, before you start — every point is one of these and there is nothing else. Each one runs your code; it is not a search for keywords.

A field that was not sent is left alone+25Patch versus replace
null clears the field, undefined does not+25Patch versus replace
replace drops what was not sent+20Patch versus replace
patchRow updates one row and copies the rest+20Not mutating your inputs
changedFields ignores a patch that changes nothing+10Patch versus replace

What it teaches

Patch versus replacebeginner

A field that was not sent and a field sent as null are different instructions — leave it alone, versus clear it — and collapsing them is how an update to one field quietly wipes another.

Not mutating your inputsintermediate

sort() and reverse() change the array you gave them, which is how a filter quietly reorders the caller's data — copy first, and the bug never exists.

The rest of The Write Side

The Double ChargeBeginner · 15 min The Lost UpdateIntermediate · 20 min Orphans and TombstonesIntermediate · 20 min All Or NothingAdvanced · 25 min