verbose code that pivots an array of records in a very unperformant and naive way.

pivot_table.js

const rdate = () => new Date(parseInt(Math.random() * Number.MAX_SAFE_INTEGER));
const data = [
		{ Symbol: 'a', Data: 0.0, At: rdate() },
		{ Symbol: 'b', Data: 0.0, At: rdate() },
		{ Symbol: 'c', Data: 1.0, At: rdate() },
		{ Symbol: 'a', Data: 1.0, At: rdate() },
		{ Symbol: 'c', Data: 0.0, At: rdate() },
		{ Symbol: 'b', Data: 1.0, At: rdate() },
		{ Symbol: 'd', Data: 0.0, At: rdate() },
];

const muhPivotTable = [
	...data
		.reduce(
			(acc, cur) =>
				((acc) => ({
					columns: acc.columns,
					// Append a pivoted version of record to at the index
					index: acc.index.set(cur['At'], [
						...(acc.index.get(cur['At']) ?? []),
						[...acc.columns.keys()].reduce(
							(o: Record<string, any | null>, k: string) => ({
								...o,
								...{ [k]: cur['Data'] ?? null },
							}),
							{ At: cur['At'] },
						),
					]),
				}))(
					acc.columns.has(cur['Symbol'])
						? acc
						: // If we haven't see this column before add it to the unique columns and
							// add nulls for the current column key.
							{
								columns: acc.columns.add(cur['Symbol']),
								index: new Map(
									[...acc.index.entries()].map(([k, vv]) => [
										k,
										vv.map((v: Record<string, any>) => ({
											...v,
											[cur['Symbol']]: null,
										})),
									]),
								),
							},
				),
			{ columns: new Set(), index: new Map() },
			// Do your aggregation of collisions here, I'm just taking the first one.
		)
		.index.values(),
].reduce((acc, cur, i) => (i === 0 ? cur : acc), null);