Simple JavaScript observation

watch.js

// Returns [get, set]. Changes to the value can be observed with onChange.
function watch(value) {
    let val = value;

    function get() {
        return val;
    }
    get.handlers = [];

    function set(newValue) {
        const old = val;
        val = newValue;
        if (old !== newValue) {
            for (let h of get.handlers) {
                h(newValue);
            }
        }
    }

    return [get, set];
}

// Handler is called when the value backed by getter changes.
function onChange(getter, handler) {
    getter.handlers.push(handler);
}

// Creates an array of watched values. Returns [getAll, append, remove]
//
// getAll() returns the array of values.
// append() returns [get, set]
// remove(i) removes the i-th value
function watchN() {
    const observations = [];

    function getAll() {
        return observations.map(get => get());
    }

    function append(value) {
        const [get, set] = watch(value);
        observations.push(get);
        return [get, set];
    }

    function remove(i) {
        values.splice(i, 1);
    }

    return [getAll, append, remove];
}