如何在Node.js中使用Promise對API進行遞歸請求？

Question

我需要從API讀取數據，該數據每次查詢僅提供100個結果，以及從何處獲取下一個100的時間戳。

我設法用下面的代碼一個接一個地執行多個請求，但是由於某種原因，它從未返回到最初的承諾。 它被卡在“不再需要獲取任何訂單”上。

app.get('/test', (req, res) => {

  const getOrders = (from) => {
    return request(mcfApiUrl + "changes?created_after_ts="+from+"&key="+mcfKey)
    .then(xml => convert.xmlDataToJSON(xml,{explicitArray:false,mergeAttrs:true}))
    .then(orders => checkForMore(orders));
  }

  const checkForMore = (orders) => {
    return new Promise((resolve, reject) => {
      if (orders['Orders']['orders'] == 100){
        getOrders(orders['Orders']['time_to']);
        console.log("Fetched "+ orders['Orders']['orders']+" orders");
        console.log("More orders available from: "+moment(orders['Orders']['time_to']*1000).format());
      }
      else {
        console.log("Fetched "+ orders['Orders']['orders']+" orders");
        console.log("No more orders to fetch");
        resolve(orders);
      }
    });
  };

  var fromdate = 1483999200;

  getOrders(fromdate)
  .then(output => res.send("Done")) // It never gets here
  .catch(err => console.log(err));

});

我想念什么？

Answer 1

您的問題是您沒有解決所有選項的checkForMore承諾。

const checkForMore = (orders) => {
    return new Promise((resolve, reject) => {
      if (orders['Orders']['orders'] == 100){
        getOrders(orders['Orders']['time_to']); // <-- not resolved
      }
      else {
        resolve(orders);
      }
    });
  };

只需將對getOrders的調用包裝為resolve解決此問題。

resolve(getOrders(orders['Orders']['time_to']))

但是，您實際上並不需要創建新的承諾：

const checkForMore = (orders) => 
  orders['Orders']['orders'] == 100
    ? getOrders(orders['Orders']['time_to'])
    : Promise.resolve(orders);

實際上，您的整個功能可以縮小為幾行：

const getOrders = (from) => 
  request(mcfApiUrl + "changes?created_after_ts="+from+"&key="+mcfKey)
    .then(xml => convert.xmlDataToJSON(xml,{explicitArray:false,mergeAttrs:true}))
    .then(orders => 
      orders.Orders.orders == 100
        ? getOrders(orders.Orders.time_to)
        : Promise.resolve(orders)
    );

現在，如果要累積所有訂單，則需要通過遞歸級別維護某些狀態。

您可以使用全局狀態或其他參數來執行此操作：

const getOrders = (from, allOrders = []) => 
  //                     ^ accumulation container
  request(mcfApiUrl + "changes?created_after_ts="+from+"&key="+mcfKey)
    .then(xml => convert.xmlDataToJSON(xml,{explicitArray:false,mergeAttrs:true}))
    .then(orders => {
      allOrders.push(orders); // <-- accumulate
      return orders.Orders.orders == 100
        ? getOrders(orders.Orders.time_to, allOrders) // <-- pass through recursion
        : Promise.resolve(allOrders)
    });