Node.js无法兑现承诺（承诺{ <pending> }）

Question

I m new in asynchronous coding

I m using csvtojson library and I'm trying to convert a csv file and pass the result in an other module .

The convert() function looks like this:

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);
    })
  });
}

My call:

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

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

When I execute the code I can see that the promise is still on pending state:

Promise { <pending> }

I think that I have to use the .then() function but I don't know where or how.

Answer 1

From converter function you are getting promise, and that object has method then . You should do something like this.

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

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

Here you can find nice tutorial about Promises, and here is documentation for Promises.

Answer 2

Promise has a fixed syntactical architecture. I'll explain it with a simple code.

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.
});

So , your function should go something like-

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.
});