callback.js

// What is `callback`?

/**
 * Async working method with callback
 * 
 * @param {*} data
 * @param {function} callback
 * 
 * @returns {string}
 */
function asyncWorkingMethod(data, callback) {
    
    // Async operation
    setTimeout(function () {
        // call back
        callback();
    }, 1000);
}

console.log('before calling asyncWorkingMethod');

var asyncWorkingMethodResult = asyncWorkingMethod({foo: 'bar'}, function () {
    console.log('callback code run HERE');
});

console.log('result of asyncWorkingMethod: ', typeof asyncWorkingMethodResult);

console.log('after calling asyncWorkingMethod');