javascript - Node.Js API 等待 dynamodb 调用完成以发送到浏览器

Question

我有大量复杂的代码用于 dynamodb 调用数据库。基本上，我有几个数据库调用相互链接，然后最重要的是我需要循环访问多个项目。然后最后我需要能够向前端发送一个新的 JSON 对象以正确显示。问题是映射在数据库调用完成之前完成，因此“newItems”变量未定义。基本上，我需要 map 等待数据库完成，然后再进行下一次迭代。

代码片段不是完整的调用，但它是相关区域。这也全部包含在 app.get() 中。

        const newItems = items.map(function(item, i) {
          dynamoDb.scan({
            TableName: GIVEAWAY_ENTRIES_TABLE,
            FilterExpression: "giveawayId = :giveawayId",
            ExpressionAttributeValues: {
              ":giveawayId": item.id
            }
          }, (error, ge_result) => {
            dynamoDb.scan({
              TableName: USERS_TABLE,
              FilterExpression: "sessionId = :sii",
              ExpressionAttributeValues: {
                ":sii": sessionId
              }
            }, (error, result) => {
              dynamoDb.scan({
                TableName: GIVEAWAY_ENTRIES_TABLE,
                FilterExpression: "giveawayId = :giveawayId and userId = :userId",
                ExpressionAttributeValues: {
                  ":giveawayId": item.id,
                  ":userId": result.Items[0].id
                }
              }, (error, gemc_result) => {

                console.log("RESPONSEEEEEEE");
                return {
                  id: item.id,
                  title: item.title,
                  thumbnail: item.thumbnail,
                  photo: item.photo,
                  description: item.description,
                  myCount: gemc_result.Count,
                  totalCount: ge_result.Count
                }
              });
            });
          })

          console.log("END OF MAPPPPP");

        });

        console.log("#######", newItems);
        res.json({'success': true, data: newItems});

Answer 1

我推荐你的第一件事是使用原生函数promisify使AWS sdk函数返回一个promise。它更具可读性!然后你可以利用node.js的promise来处理异步代码，你可以这样使用它:

const util = require('util');
const safePromisify = function (fun, methodsArray) {
  const suffix = 'Async';
    methodsArray.forEach(method => {
      fun[method + suffix] = util.promisify(fun[method]);
  });
}
safePromisify(dynamoDb, ['scan']);

const newItems = items.map(function async(item, i) {
//                                  ^^^^^ this is required to use await

// Note the async and await keywords
  let ge_result = await dynamoDb.scanAsync({
    TableName: GIVEAWAY_ENTRIES_TABLE,
    FilterExpression: "giveawayId = :giveawayId",
    ExpressionAttributeValues: {
      ":giveawayId": item.id
    }
  });

  let result = await dynamoDb.scanAsync({
    TableName: USERS_TABLE,
    FilterExpression: "sessionId = :sii",
    ExpressionAttributeValues: {
      ":sii": sessionId
    }
  });

  let gemc_result = await dynamoDb.scanAsync({
    TableName: GIVEAWAY_ENTRIES_TABLE,
    FilterExpression: "giveawayId = :giveawayId and userId = :userId",
    ExpressionAttributeValues: {
      ":giveawayId": item.id,
      ":userId": result.Items[0].id
    }
  });

  return {
    id: item.id,
    title: item.title,
    thumbnail: item.thumbnail,
    photo: item.photo,
    description: item.description,
    myCount: gemc_result.Count,
    totalCount: ge_result.Count
  }
});

Note: You have to append the word Async on the function call