javascript - 调用一个包含来自另一个函数的 Promise 的函数

Question

我正在尝试学习如何在 NodeJs 中使用 Promise，并且正在使用 AWS-SDK 库来访问 S3 对象。我的目标是从 init() 函数中调用 readFromS3() 函数并打印出文件的内容。但是，我没有得到我想要的结果，如下所示，init() 中的第一个 console.log 语句。我知道 promise 并不完整，并且希望获得您关于如何阻止执行直到 news 对象不为空的建议？？

const AWS = require('aws-sdk');

const S3 = new AWS.S3({});

const CONFIG = {
  init() {
    const news = CONFIG.readFromS3();
    console.log('These are the file contents ' + JSON.stringify(news));
    console.log('THIS IS THE END. This should only print after news have been read from S3');
  },

  readFromS3() {
    // set parameters for reading S3 files
    const options = {
      Bucket: 'my-bucket',
      Key: 'myFile.txt'
    };

    // create a promise to read from S3
    const readS3Promise = S3.getObject(options).promise();

    // start reading from s3
    readS3Promise
      .then(function(data) {
        return JSON.parse(data.Body);

        });
      })
      .catch(function(error) {
        console.log('ERROR: Cannot read from S3');
        throw error;
      });
  }
};

CONFIG.init();

但是，不幸的是，我目前的输出是这样的:

These are the SPECS undefined
THIS IS THE END. This should only print after news have been read from S3
{... // JSON data from S3 printed out

Answer 1

您需要从 read 方法返回您的 promise ，并在 init 函数内等待该调用，以便获得输出。像这样的东西

const AWS = require('aws-sdk');

const S3 = new AWS.S3({});

const CONFIG = {
  async init() {
    const news = await CONFIG.readFromS3();
    console.log('These are the file contents ' + JSON.stringify(news));
    console.log('THIS IS THE END. This should only print after news have been read from S3');
  },

  readFromS3() {
    // set parameters for reading S3 files
    const options = {
      Bucket: 'my-bucket',
      Key: 'myFile.txt'
    };

    // create a promise to read from S3
    const readS3Promise = S3.getObject(options).promise();

    // start reading from s3
    return readS3Promise
      .then(function(data) {
        return JSON.parse(data.Body);

        });
      })
      .catch(function(error) {
        console.log('ERROR: Cannot read from S3');
        throw error;
      });
  }
};

CONFIG.init();