Imperative.js uses javascript generators to reproduce the advantages of React - reusable, stateful components - without the complexity or code size. It is an almost trivial library of only 1.5kb (minified + gzipped) but no less general than React, and actually more ergonomic in some cases, such as waiting for a fetch call.

An imperative component is an ordinary javascript generator. It can be run by calling run. The optional second argument to run will set the root of the application (defaults to document.body):

let { run } = require("imperative");
run(function*() { yield* H('div', "Hello, world"); });

Instead of JSX, Imperative uses ordinary javascript functions. These can be easily remembered with the mnemonic HASTE. H - HTML element, A - Attribute, S - Style, T - text, E - events.

function* example() {
    // the first argument to H is an element name, after that pass
    // any number of components or arrays of components.
    yield* H('div',
        // all the HASTE functions return components. This one will set a style
        // on the parent div.
        S('backgroundColor', 'black'),
        // alternative style syntax with objects
        S({'color': 'white'}),
        function*() {
            // the E component will wait for the given event, then return the
            // event object.
            let ev = yield* E('click');
            yield* S('border', '1px solid green');
        },
        T('Example Div'));
}

Imperative uses normal generator control flow. The children of an H call will run in parallel until one of them returns, the return value of that child will return from H.

function*() {
    let color = yield* H('div',
        H('button', T('Red'), function*() { yield* E('click'); return 'Red'; }),
        H('button', T('Blue'), function*() { yield* E('click'); return 'Blue'; }));
    yield* H('div', `You chose the ${color} pill`);
}

Sometimes you want the parallel execution without introducing a DOM parent element. This can be accomplished with multi.

function*() {
    let fetchResult = yield* multi(
        // Fetch = fetch wrapped into an imperative component
        Fetch('/api'),
        H('div', T('Waiting for api...')));
    let jsonResult = yield* multi(
        fetchResult.json(),
        H('div', T('Waiting for response body...')));
    yield* H('div', T(JSON.stringify(jsonResult)));
}

Fetch simply wraps fetch into an imperative generator, adding auto-cancellation. The json and text methods of the response are also wrapped. wait is a similar wrapper for setTimeout, and waitFrame for requestAnimationFrame.

Many UI problems can be solved using a combination of control flow and parallel execution. Sometimes we do need a mechanism to communicate between different pieces of the UI. For this purpose we can use Var. Var has methods:

get - get current value
set - set a new value
next - component, will return with next value
fmap - create a new component with the provided function, each time the value changes

function*() {
    let color = Var('red');
    yield* H('div',
        H('button', T('red'), function*() {
            while(true) {
                yield* E('click');
                color.set('red');
            }
        }),
        H('button', T('blue'), function*() {
            while(true) {
                yield* E('click');
                color.set('blue');
            }
        }),
        H('div', color.fmap(current => T(`you chose the ${current} pill`))));
}

These functions - run, HASTE, Var, multi, Fetch, wait - are the high-level API of imperative. For low-level operations you need to understand what the generators are doing. An imperative component is a generator that yields functions of the form {H, cleanup} => Promise. The parent will wait for the promise, then return its result to the generator. cleanup allows you to register cleanup functions which will run when the generator finishes, or when it is cut off by a parallel generator finishing. H in this context is different from the main H, in a yield function the H has signature DomElement => DomElement and allows you to access the parent dom element directly.

Here is an example of using the low-level API to implement intersection observers.

function* visible(threshold) {
    return yield ({H, cleanup}) => new Promise(resolve => {
        H(elem => {
            let ob = new IntersectionObserver(([entry]) => {
                if(entry.isIntersecting) {
                    resolve(entry);
                }
            }, {threshold}).observe(elem);
            cleanup(() => ob.disconnect());
        })
    });
}

Another low-level function is local. local allows you to change the set of options being passed down into each generator. The HTML DOM-specific API of imperative is implemented on top of multi and local, and it is equally easy to use any other type of persistent UI tree, e.g. within a canvas element.