index.js
function addInterfaceMethod(interfaceName, funcName, fN) {

    // Store a reference to every interface/method
    if (Steam.INTERFACES[interfaceName] === undefined) {
        Steam.INTERFACES[interfaceName] = {};
    }
    Steam.INTERFACES[interfaceName][funcName] = fN;

    // Camel case the method name
    var name = funcName.substr(0,1).toLowerCase() + funcName.substr(1);

    // Add method to Steam's prototype
    if (!isMultiGameInterface(interfaceName)) {
        steam[name] = fN;
    }
    // If multiple interfaces use the same method name
    // Create a new method that requires a gameid property to find the correct method
    //  and call the steam method automatically
    else {
        // We only need to do this once
        if (steam[name] !== undefined) return;

        // e.g. Turns 'IEconItems_440' into 'IEconItems'
        var multi_interface_name = interfaceName.split('_')[0];

        // Add method to Steam's prototype
        steam[name] = function(steamObj, callback) {
            var gameid = get(this, steamObj, 'gameid');

            var interface_name = multi_interface_name + '_' + gameid;

            Steam.INTERFACES[interface_name][funcName].call(this, steamObj, callback);
        };
    }

}
function addInterfaceMethod(interfaceName, funcName, fN) {

    // Store a reference to every interface/method
    if (Steam.INTERFACES[interfaceName] === undefined) {
        Steam.INTERFACES[interfaceName] = {};
    }
    Steam.INTERFACES[interfaceName][funcName] = fN;

    // Camel case the method name
    var name = funcName.substr(0,1).toLowerCase() + funcName.substr(1);

    // Add method to Steam's prototype
    if (!isMultiGameInterface(interfaceName)) {
        steam[name] = fN;
    }
    // If multiple interfaces use the same method name
    // Create a new method that requires a gameid property to find the correct method
    //  and call the steam method automatically
    else {
        // We only need to do this once
        if (steam[name] !== undefined) return;

        // e.g. Turns 'IEconItems_440' into 'IEconItems'
        var multi_interface_name = interfaceName.split('_')[0];

        // Add method to Steam's prototype
        steam[name] = function(steamObj, callback) {
            var gameid = get(this, steamObj, 'gameid');

            var interface_name = multi_interface_name + '_' + gameid;

            Steam.INTERFACES[interface_name][funcName].call(this, steamObj, callback);
        };
    }

}
Add some easy references to the new method