ScriptdrillCount by a field
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
Return an object that maps each value of `item[key]` to how many items have it. Skip items where the field is missing or `undefined`.
What this exercise teaches
`(counts[k] || 0) + 1` starts a new count at 1 and increases existing ones. Skipping undefined keeps a stray "undefined" key out of the result.
A hint if you get stuck
Loop once. For each item read `item[key]`, skip `undefined`, and add 1 to the count, starting from 0 the first time.
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