javascriptnode.jstypescriptpromisefs

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!


Solution

  • 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!
      });
    }