javascript promises definition & usage

Question

I am really struggling with integrating Javascript promises into my code and would be grateful for some help on this.

I have this simple function which is part of a program that handles creating, reading, updating and deleting of files, so I decided to change it from using callbacks to using promises.

this is the delete function:

lib.delete = (dir, file, callback) => {
  /* unlink the file */
  fs.unlink(lib.baseDir + dir + '/' + file + '.json', (err) => {
    if (!err) {
      callback(false);
    } else {
      callback(' Error deleting file');
    }
  });
};

The way I was going to do this, was to enclose the async part in a promise:

lib.delete = (dir, file, callback) => {
  /* unlink the file */
  return new Promise((resolve, reject) => {
    fs.unlink(lib.baseDir + dir + '/' + file + '.json', (err) => {
      if (!err) {
        resolve(false);
      } else {
        reject(err);
      }
    });
  });
});

But this all seems wrong because here I'm defining the promise, but here is actually where I actually want to use it, i.e. deal with the result of it with .then.

I am missing something basic here, I have this feeling that I need to change the way I am thinking about the all delete function... Any help would be appreciated.

Answer 1

If you want to use Promise you don't have pass a callback into these arguments, it's the goal of promise : call any function you want after resolve/reject.

For example :

With callback :

lib.delete = (dir, file, callback) => {
  /* unlink the file */
  fs.unlink(lib.baseDir + dir + '/' + file + '.json', (err) => {
    if (!err) {
      callback(false);
    } else {
      callback(' Error deleting file');
    }
  });
};

lib.delete('./dir', 'filname.txt', function(res){
  console.log(res);
})

With promise

lib.delete = (dir, file) => {
  return new Promise((resolve, reject) => {
    fs.unlink(lib.baseDir + dir + '/' + file + '.json', (err) => {
      if (!err) {
        resolve(false);
      } else {
        reject(err);
      }
    });
  });
});

lib.delete('./dir', 'filname.txt')
  .then( () =>  console.log("No error i'm happy")
  .catch( (err) =>  console.log('Error', err)

then : exec when resolve is trigger

catch : exec when reject is trigger

I hope it's gonna help you !