Comment by emmanueloga_

For those that have not implemented toposort or don't remember it: 1) only directed graphs without cycles can be topologically sorted (DAGs) 2) there can be more than one topological order and 2) reverse post order of depth first traversal from every unvisited node shields a topological order.

In JavaScript:

    function toposort(graph = { a: ['b', 'c'], b: ['d'] }) {
      const order = [];
      const visited = new Map();
      
      function dfs(node) {
        const status = visited.get(node);

        if (status === 1) throw new Error("Cycle found.");
        if (status === 2) return;

        visited.set(node, 1); // In progress.
        const adjacent = graph[node] ?? [];
        for (const neighbor of adjacent) dfs(neighbor);
        visited.set(node, 2); // Done.

        order.unshift(node); // Reverse post order.
      }

      for (const node in graph) {
        if (!visited.has(node)) dfs(node);
      }

      return order;
    }