ScriptdrillPivot rows into a lookup
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
Turn flat rows into a two-level lookup: `result[row[rowKey]][row[colKey]]` is the sum of `row[valueKey]` over every row with that pair. Pairs that never appear are absent. A sum of 0 is still kept.
What this exercise teaches
Two levels of "create when missing" build the nested object in one pass, and adding instead of assigning sums duplicate pairs. Starting from 0 with `?? 0` keeps sums that cancel out to 0.
A hint if you get stuck
For each row, create `out[r]` as `{}` when missing, then add the value to `out[r][c]`, starting from 0.
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