How to return a promise when writestream finishes? [duplicate]

问题

This question already has answers here:

How do I convert an existing callback API to promises? (20 answers)

Closed 3 years ago.

I have such a function, which create a write stream and then write the string array into the file. I want to make it return a Promise once the writing is finished. But I don't know how I can make this work.

function writeToFile(filePath: string, arr: string[]): Promise<boolean> {
   const file = fs.createWriteStream(filePath);
   arr.forEach(function(row) {
     file.write(row + "\n");
   });
   file.end();
   file.on("finish", ()=>{ /*do something to return a promise but I don't know how*/});
}

Thank you for any comment!

回答1:

You'll want to use the Promise constructor:

function writeToFile(filePath: string, arr: string[]): Promise<boolean> {
  return new Promise((resolve, reject) => {
    const file = fs.createWriteStream(filePath);
    for (const row of arr) {
      file.write(row + "\n");
    }
    file.end();
    file.on("finish", () => { resolve(true); }); // not sure why you want to pass a boolean
    file.on("error", reject); // don't forget this!
  });
}

回答2:

You need to return the Promise before the operation was done.
Something like:

function writeToFile(filePath: string, arr: string[]): Promise<boolean> {
    return new Promise((resolve, reject) => {
        const file = fs.createWriteStream(filePath);
        arr.forEach(function(row) {
            file.write(row + "\n");
        });
        file.end();
        file.on("finish", () => { resolve(true) });
    });
}

来源：https://stackoverflow.com/questions/39880832/how-to-return-a-promise-when-writestream-finishes