hot-reload.js

const filesInDirectory = (dir) => new Promise (resolve =>

    dir.createReader ().readEntries ((entries) =>

        Promise.all (entries.filter ((e) => e.name[0] !== ".").map (e =>

            e.isDirectory
                ? filesInDirectory (e)
                : new Promise ((resolve) => e.file (resolve))
        ))
        .then ((files) => [].concat (...files))
        .then (resolve)
    )
);

const timestampForFilesInDirectory = (dir) =>
        filesInDirectory (dir).then (files =>
            files.map (f => f.name + f.lastModifiedDate).join ());

const reload = () => {

    chrome.tabs.query ({ active: true, currentWindow: true }, tabs => {

        if (tabs[0]) { chrome.tabs.reload (tabs[0].id) }

        chrome.runtime.reload ();
    });
};

const watchChanges = (dir, lastTimestamp) => {

    timestampForFilesInDirectory (dir).then (timestamp => {

        if (!lastTimestamp || (lastTimestamp === timestamp)) {

            setTimeout (() => watchChanges (dir, timestamp), 1000); // retry after 1s

        } else {

            reload ();
        }
    });

};

chrome.management.getSelf ((self) => {

    if (self.installType === "development") {

        chrome.runtime.getPackageDirectoryEntry ((dir) => watchChanges (dir));
    }
});