ScriptdrillLeft join two lists
A JavaScript exercise from Scriptdrill. Solve it with a code editor in your browser that runs your function against example and hidden tests, and it is checked the moment you submit.
The task
For each row of `left`, in order, return a new object combining it with the first row of `right` that has the same `row[key]` (fields from `right` win when both have them). A left row without a match is returned as a copy. Keys match only when they are the same value, so `1` does not match `"1"`. Do not change either list.
What this exercise teaches
The Map turns the lookup into one step per left row instead of a scan of right, and Map keys keep their type. Only setting a key the first time keeps the first match. Spreading into new objects leaves both inputs untouched.
A hint if you get stuck
Index `right` once in a `Map` from key to the first row with that key (skip keys you already have). Then map `left` to `{ ...l, ...match }` or `{ ...l }`.
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