Question

我编写了以下代码来完成这项工作...但是它只调用一次。但是我想多次调用该列表中的所有对象。是否可以在nodejs中同步调用任何代码。

    async function processObjs(){
          var response = "done";
        for(var i = 0; i < objectsToProcess.length; ++i) {
            var obj = objectsToProcess[i];
            await processObject(obj);
            console.log("done for : " + obj);
        }
        return response;
      }

      function processObject(objectType){
        var filePath = objectType+".csv";
        return newPromise(objectType,filePath);
      }

      function newPromise(objectType,filePath){
          return new Promise(function (resolve){
      //following code should run synchronously
            dataStore= new DataLakeStore(objectType);
            let firstPage = true;
            console.log("start processing for object :"+objectType);
            dataStore.search(qry).forEachPage(page => {
                const jsonToWrite = page.hits.map(record => {
                  return record._source.doc;
                });
                let csvData;
                let flag;
                if (firstPage) {
                  firstPage = false;
                  flag = "ax+";
                  csvData = json2csv(jsonToWrite);
                }else{
                  flag = "a";
                  csvData = json2csv(jsonToWrite, { header: false }) + "\n";
                }
                fs.writeFileSync(filePath, csvData, { flag });
              });
              resolve("DONE");
          });

      }

      processObjs().then(function (response) {
        console.log("Completed FOR All " + response);
      })

Answer 1

您可以在Node.js中同步运行一些代码，例如，您可以使用fs.readFileSync同步读取文件。但是您在大多数情况下应该，请记住，大多数Node.js I / O都是异步的。

大多数API使用此异步模式，以确保函数调用不会阻塞。对于这些函数调用，例如上面的示例，您无法同步运行它们，但是可以使用asynch / await模式为您提供同步函数将为您提供的相同语法。

// Simulate say a DB or REST call here.. 
function asynchReadSomeData(callback) {
    setInterval(() => {
        callback("OK");
    }, 2000);  
}

function asynchReadSomeDataPromise() {
    return new Promise((resolve) => {
        asynchReadSomeData((result) => resolve(result));
    });
}

async function testAwait() {
    /* We can use very similar syntax to a synchonous function call here..*/
    console.log("Calling asynchReadSomeDataPromise, waiting for result...");
    let result = await asynchReadSomeDataPromise();
    console.log("Result: ", result);
}

testAwait();

Answer 2

您可以使用async和await通过同步方式获得结果。

 async function processObject(objectType){
        var filePath = objectType+".csv";
        return await newPromise(objectType,filePath);
      }

  function newPromise(objectType,filePath){
      return new Promise(function (resolve){
  //following code should run synchronously
        dataStore= new DataLakeStore(objectType);
        let firstPage = true;
        console.log("start processing for object :"+objectType);
        dataStore.search(qry).forEachPage(page => {
            const jsonToWrite = page.hits.map(record => {
              return record._source.doc;
            });
            let csvData;
            let flag;
            if (firstPage) {
              firstPage = false;
              flag = "ax+";
              csvData = json2csv(jsonToWrite);
            }else{
              flag = "a";
              csvData = json2csv(jsonToWrite, { header: false }) + "\n";
            }
            fs.writeFileSync(filePath, csvData, { flag });
          });
          resolve("DONE");
      });

  }
(async () => {
   const result = await processObjs();
   console.log(result)
})();

如何在NodeJS中同步调用异步代码

2 个答案: