Interview example

Company.js

// Note: Save as Company.js and run 'babel-node Company.js' to display in the command window. This is written in ES2015
const Company = ((window) => {
    // Store an array of callbacks
    const _callbacks = [];

    // Store whether the 'init' function has already been called
    let _isInitialised = false;

    function init() {
        _isInitialised = true;

        while (_callbacks.length > 0) {
            // This is the same as using a queue, where the first callback enqueued is dequeued etc...
            const callback = _callbacks.shift();

            // Call the callback
            callback(); // Could use callback.call(); to change the context
        }
    }

    function ready(...callbacks) {
        // If 'init' has already been called, then just call the array of 'callbacks'
        if (_isInitialised) {
            for (const callback of callbacks) {
                window.console.log('Ready after initialisation');

                // Call the callback
                callback(); // Could use callback.call(); to change the context
            }

            return;
        }

        // Add the callback arguments to the array, but only if a function type
        for (const callback of callbacks) {
            if (Object.prototype.toString.call(callback) === '[object Function]') {
                window.console.log('Ready before initialisation');
                _callbacks.push(callback);
            }
        }
    }

    // Public API
    return {
        init: init,
        ready: ready
    };
})(window);

// Example, with private scope

((window, global) => {

    function example1() {
        window.console.log('Example function => 1');
    }

    function example2() {
        window.console.log('Example function => 2');
    }

    function example3() {
        window.console.log('Example function => 3');
    }

    function example4() {
        window.console.log('Example function => 4');
    }

    // Add three callback functions to be called when 'init' is invoked
    global.ready(example1, example2, example3);

    // After 3 seconds, call the 'init' function
    window.setTimeout(() => {
        global.init();

        window.console.log('Init finally invoked...');

        // This will execute immediately due to 'init' having already been called
        global.ready(example4);
    }, 3000);

    console.log('Lets wait for 3 seconds...');

})(window, Company); // Pass 'Company' to the IIFE