background.js

/*---------------------------------------------------------------
>>> BACKGROUND
-----------------------------------------------------------------
# Global variable
# Localization
# Listeners
    # Storage change
    # Message
---------------------------------------------------------------*/

/*--------------------------------------------------------------
# GLOBAL VARIABLE
--------------------------------------------------------------*/

var extension = {
    locale: {},
    storage: {}
};


/*--------------------------------------------------------------
# LOCALIZATION
--------------------------------------------------------------*/

function getLocalization(code) {
    var language = code || navigator.language;

    if (language.indexOf('en') === 0) {
        language = 'en';
    }

   fetch(chrome.runtime.getURL('_locales/' + language + '/messages.json')).then(function (response) {
        if (response.ok) {
            response.json().then(function (response) {
                extension.locale = {};

                for (var key in response) {
                    extension.locale[key] = response[key].message;
                }
            });
        } else {
            fetch(chrome.runtime.getURL('_locales/en/messages.json')).then(function (response) {
                if (response.ok) {
                    response.json().then(function (response) {
                        extension.locale = {};

                        for (var key in response) {
                            extension.locale[key] = response[key].message;
                        }
                    });
                }
            });
        }
    });
}


/*---------------------------------------------------------------
# LISTENERS
---------------------------------------------------------------*/

/*--------------------------------------------------------------
# STORAGE CHANGE
--------------------------------------------------------------*/

chrome.storage.onChanged.addListener(function (changes) {
    for (var key in changes) {
        extension.storage[key] = changes[key].newValue;
    }

    getLocalization(extension.storage.language);
});


/*---------------------------------------------------------------
# MESSAGE
---------------------------------------------------------------*/

chrome.runtime.onMessage.addListener(function (message, sender, sendResponse) {
    if (message === 'get-locale') {
        var response = extension.locale;

        sendResponse(response);

        return response;
    } else if (message === 'get-tab-url') {
        var response = {
            url: new URL(sender.tab.url).hostname,
            id: sender.tab.id
        };

        sendResponse(response);

        return response;
    }
});


/*--------------------------------------------------------------
# INITIALIZATION
--------------------------------------------------------------*/

chrome.storage.local.get(function (items) {
    extension.storage = items;

    getLocalization(items.language);
});