What does this mean in the Observable TC-39 proposal?

In the TC-39 Observable API proposal I see:

Compositional: Observables can be composed with higher-order combinators.

A combinator is a function that can be used to combine two (or more?) logic pieces into single, more useful construct.

So what does the above quote mean in the context of observables?

Solution

Combinators, in the context of Observables, refer to the functions (usually attached to Observable.prototype, in the implementations I've seen) that allow you to transform one kind of Observable into another. There's a good example a little bit further down in the proposal:

// Return an observable of special key down commands
function commandKeys(element) {
    let keyCommands = { "38": "up", "40": "down" };

    return listen(element, "keydown")
        .filter(event => event.keyCode in keyCommands)
        .map(event => keyCommands[event.keyCode])
}

In this example, filter and map are the combinators - they work in a similar fashion to the functions of the same name on Array.prototype. The important thing to note is that, similarly to the array functions, they always return a new Observable - they do not mutate the existing one. This is useful, because it means that further combinators could be chained on to the result of commandKeys if necessary.