Observer on input value triggers the observer callback too soon

[emiltamas]

JSFiddle here

The observer callback probably gets fired internally when Ractive sets the value to an empty string after being initially undefined. Is this by design? My expectation is that the observer shouldn't trigger when the variable is being initialised.

[Rich-Harris]

Yes, this is actually part of the design - observer callbacks always fire immediately unless you pass in an options object with {init: false}:

var observer = ractive.observe( 'foo', function ( foo ) {
  // this will NOT be called immediately
}, { init: false });

I wrote a bit about the rationale on this thread - will quote the relevant bit:

As for the init: true default behaviour, browniefed hits the nail on the head - it's not actually an event system, but a data flow system. The mechanism is the same, but the idea is subtly different - it's closer to the world of RxJS and Bacon.js than traditional pub/sub (which in my experience scales poorly), but without using Rx/Bacon primitives. It's designed to enable a form of reactive programming, where any given input state can lead to exactly one output state - to achieve that kind of idempotency the observer has to be called with the initial value.

With pub/sub based systems it's not uncommon to see stuff like this:

// set up some form of data-binding...
model.on( 'change:foo', function ( foo ) {
  view.renderFoo( foo );
});

// ...and initialise to the current value
view.renderFoo( model.get( 'foo' ) );

Observers avoid that kind of redundancy (and because of their clearer intent, it's easier to guard against things like circularity). But of course it's sometimes desirable to use observers in other ways, which is why the init option exists.

[emiltamas]

You, Sir, are amazing! That makes sense. I now also see the init option in the documentation.
I initially missed it because it was light-greyed out as an inline comment in the sample source.