CODEWITHSUNDEEP
javascriptnode.jstypescriptpromisefs
Lubor
Lubor

Reputation: 999

How to return a promise when writestream finishes?

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!

Upvotes: 17

Views: 24153

Answers (3)

Patrick Hund
Patrick Hund

Reputation: 20236

With more recent Node.js versions, you can use the utility function finished from the stream package, like so:

import { finished } from 'node:stream/promises';
import { createWriteStream } from 'node:fs';

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

Upvotes: 1

Bergi
Bergi

Reputation: 664538

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!
  });
}

Upvotes: 38

Nitzan Tomer
Nitzan Tomer

Reputation: 164139

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) });
    });
}

Upvotes: 3

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