可以用Promise實現。每個任務的結果用一個Promise保存，如果A依賴B則在B resolve之后才開始A.

const tasks = {

  a: {

    dep: ["b", "c"],

    run: (resultB, resultC) => resultB + resultC

  },

  b: {

    run: () => `B`

  },

  c: {

    dep: ["d"],

    run: resultD => `C` + resultD

  },

  d: {

    run: () => `D`

  },

};

const targetStack = [];

function run(taskTree, target, results = {}) {

  if (targetStack.indexOf(target) !== -1) {

    const err = new Error(`recursive dependicies: ${targetStack.concat([target]).join(" <- ")}`);

    targetStack.length = 0;

    throw err;

  }

  targetStack.push(target);

  const deps = (taskTree[target].dep || []).map(

    depName => results[depName] = results[depName] || run(taskTree, depName, results)

  );

  targetStack.pop();

  return Promise.all(deps).then(deps => taskTree[target].run.apply(undefined, deps));

}

run(tasks, "a").then(resultA => console.log(resultA));

(這個代碼其實不需要任務是樹形，只要求是無環圖)