Flat Map Latest (through events)

flatmaplatest.js

export const VALUE = Symbol("VALUE");

const attempt = (fn, ...args) => {
    try {
        return fn(...args)
    } catch (e) {}
}

const listen = function (event, cb) {
    const eProp = Symbol.for(event);

    if (!this.hasOwnProperty(eProp)) {
        this[eProp] = [];
    }

    const listeners = this[eProp];

    if (-1 === listeners.indexOf(cb)) {
        listeners.push(cb);
    }

    return unlisten.bind(this, event, cb);
};

const unlisten = function (event, cb) {
    const eProp = Symbol.for(event);

    if (!this.hasOwnProperty(eProp)) {
        return;
    }

    const listeners = this[eProp].filter(listener => cb !== listener);

    this[eProp] = listeners;
}

const emit = function (event, payload) {
    const eProp = Symbol.for(event);

    if (!this.hasOwnProperty(eProp)) {
        return;
    }

    const listeners = this[eProp];

    listeners.forEach(cb => attempt(() => cb(payload)));
};

const makeEmitter = fn => {
    fn.on = listen.bind(fn);
    fn.off = unlisten.bind(fn);
    fn.emit = emit.bind(fn);

    return fn;
};

const makeRunner = fn => (jobId, args) => {
    return fn(...args).then(ret => ({ jobId, ret }));
};

export const flatMapLatest = fn => {
    let latestJobId = 0;

    const runner = makeRunner(fn);
    const flatMapper = makeEmitter((...args) => runner(++latestJobId, args).then(job => {
        if (job.jobId < latestJobId) {
            return;
        }

        flatMapper.emit(VALUE, job.ret);
    }, err => console.error(err)));

    return flatMapper;
}

export { fml as default };