Promise.tap

[SohumB]

It would be nice for quick debugging for a .tap function to exist, such that

.then(function(obj) { tapFn(obj); return obj })

could be replaced by

.tap(tapFn)

Thanks!

[benjamingr]

To be fair I'm not sure why we have .call and .get in the library itself. I tend to think of all these things as being 'extras'. I do see the debugging value of this (and debugging seems like pretty much the only time you'd want this since this essentially is causing a side effect).

It is of course very possible to do this from your side just like you described:

Promise.prototype.tap = function(fn){
    return this.then(function(value){
         fn(obj);
         return obj;
    });
};

[SohumB]

Fair enough. Thanks!

[spion]

I like this function...

Here is one use case:

cache.get('foo')
.then(foo => foo ? foo : db.get('foo').tap(cache.put))
.then(res.json);

vs

cache.get('foo')
.then(foo => foo ? foo : db.get('foo').then(foo => cache.put(foo).thenReturn(foo)))
.then(res.json);

Can you come up with other cases? :)

[petkaantonov]

Passing cache.put as a function reference is not the same as calling it as a method cache.put() as you know @spion :P

[tgriesser]

FWIW I use tap all the time and would find it a useful core add.

login: function(body) {
  return new User({email: body.email})
    .fetch({require: true})
    .tap(function(user) {
      return bcrypt.compareAsync(body.password, user.password);
    })
    .then(function(user) {
      return user.id
    });
}

[petkaantonov]

@tgriesser why are you calling compare but not doing anything with the result? :P

[tgriesser]

Because I don't need to do anything with it, it's purely called to ensure it doesn't throw (if the password is wrong).

[gaearon]

Yup, just today I wrote this:

  return this.lastPromise
    .bind(this)
    .then(function (result) {
      if (!this.hasOpenWidget()) {
        this.destroyWidget();
      }

      return result;
    })
    .catch(Promise.CancellationError, $.noop)
    .then(function (result) {
      return result || options.widget.DEFAULT_RESULT;
    });

Note second then returns result because it merely needs to do something else and pass it on.

[petkaantonov]

This is in 1.1