Question

我是异步编码的新手

我正在使用 csvtojson 库，我正在尝试转换csv文件并将结果传递给其他模块。

convert（）函数如下所示：

convert: function (csvFilePath) {
  return new Promise((resolve, reject) => {

    const options = { delimiter: ["|",","],
                      noHeader: true ,
                      headers: ["header1", "header2"]
                    }

    csv(options)
    .fromFile(csvFilePath)
    .on('end_parsed',(convertedJson) => {
      resolve(convertedJson);
    })
    .on("done",(error) => {
      reject(error);
    })
  });
}

我的电话：

const converter = require ("./converter")();

let json;
json = converter.convert("./importSample.csv");
console.log(json);

当我执行代码时，我可以看到承诺仍处于暂挂状态：

Promise { <pending> }

我认为我必须使用.then()函数，但我不知道在哪里或如何。

Answer 1

从converter函数获得承诺，该对象具有方法then。你应该这样做。

const converter = require ("./converter")();

converter.convert("./importSample.csv").then(json => {
  console.log(json);
}).catch(error => {
  console.log(error);
});

Here你可以找到关于Promises的精彩教程，here是Promises的文档。

Answer 2

Promise有一个固定的语法架构。我将用一个简单的代码解释它。

var x = new Promise((resolve,reject)=>{
    //here you perform an asynchronous call
    resolve(value); //receive it in 'then' part of promise
    reject(error): //if your operation fails due to any error, you call reject, which is handled by 'catch' part of the promise.
});
x.then((value)=>{
    //this is the part which was called using resolve, and the value it receives is the value you passed as argument in resolve.
});
x.catch((error)=>{
    //this part is called by reject. the error received is the value you passed inside the reject.
});

所以，你的功能应该像 -

converter.convert("./importSample.csv").then((json)=>{
    //here is your part of code which is to be handled synchronously after the promise is called.
}).catch((error)=>{
     //in case any error occurs and you want your program to exit gracefully.
});

Node.js无法履行承诺（Promise {<pending>}）

2 个答案: