Default values for do() parameters

[felixfbecker]

promise.then() and subscribe() can be called with passing undefined as the first parameter, e.g. to only handle errors or completions. do() requires to pass a noop function like () => undefined. It should just have this as default value.

doOnComplete and doOnError shorthands would also be nice.

[midnight-wonderer]

Did you know that you can safely pass null to do operator?
reference:
https://github.com/ReactiveX/rxjs/blob/01a2d15/spec/operators/do-spec.ts#L94
https://github.com/ReactiveX/rxjs/blob/01a2d15/src/Subscriber.ts#L169

[felixfbecker]

Not according to the signature:

export declare function _do<T>(this: Observable<T>, next: (x: T) => void, error?: (e: any) => void, complete?: () => void): Observable<T>;

[david-driscoll]

It's entirely possible that the signature is incorrect, however it feels like a corner case. What would someone be trying to accomplish by calling do() by itself?

As to doOnComplete and doOnError you can easily do do({ error(e) { /**/ } }); or `do({ complete() {} });

[felixfbecker]

For example to instrument a function with OpenTracing:

const span = childOf.tracer().startSpan('my operation', { childOf })
return someAction()
  // ... apply operators ...
  .do(undefined, err => {
    this.logger.error(err);
    span.setTag('error', true);
    span.log({ 'event': 'error', 'error.object': err });
  })
  .finally(() => {
    span.finish();
  })

this is personal taste of course, but I find the object version less readable:

const span = childOf.tracer().startSpan('my operation', { childOf })
return someAction()
  // ... apply operators ...
  .do({ error: err => {
    this.logger.error(err);
    span.setTag('error', true);
    span.log({ 'event': 'error', 'error.object': err });
  }})
  .finally(() => {
    span.finish();
  })

[david-driscoll]

Unfortunately I can safely say that the shorthand operators for do (and their counter parts for subscribe) probably won't be making it back into the core library. We already have enough operator overload with the current number, adding more will just make things more confusing for others.

However you could easily make your own using augmentation for your application / library. You could even make it an npm package if you wanted! 🎉

See:
https://github.com/ReactiveX/rxjs/blob/bc1e1e53897aeca61288378cc8e2b33ed20dbbcf/src/add/operator/do.ts

[kwonoj]

as same as @david-driscoll , I don't think we'll bring back variant of doOn.. but making signature of do to make next to optional seems considerable option. Only thing I'd like to have is if there's some way to make partial param must have one at least (either next, error, or complete) which seems not trivial.

[felixfbecker]

you could add overloads for all 2^3 cases. But why would you want this? Can't you just return the source if no argument is passed? Where's the benefit to restricting it?

[kwonoj]

Functionally it'll work as you'd described. Reason for this is mostly about explicitness of api definition signature - compare to subscribe doesn't have any observer contract param, do is all about triggering side effect by provided handler and would not like to implicit assumption you could do do() which actualy doesn't do anything. We're trying to make TS definition as correct api signature even JS user can refer it to clarify behavior of api.

[artaommahe]

this do(null, () => null) does not work with strictNullChecks enabled

[ts] Argument of type 'null' is not assignable to parameter of type '(x: {}) => void'.

[martinsik]

@artaommahe You can use undefined instead:

do(undefined, () => null)

[lock]

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