Trying to prepare for Robert Hall Tech test: Node.js

What are some rules of thumb with RESTful apis?

- Same result should occur whether you run a command once or 10 times - Use verbs when provding an endpoint that does a task.

How would you scale Node application

1. cloning using Cluster module. 2. decomposing the application into smaller services - i.e micro services.

Intermediate - How does Node.js Work?

A Node.js application creates a single thread on its invocation. Whenever Node.js receives a request, it first completes its processing before moving on to the next request. Node.js works asynchronously by using the event loop and callback functions, to handle multiple requests coming in parallel. An Event Loop is a functionality which handles and processes all your external events and just converts them to a callback function. It invokes all the event handlers at a proper time. Thus, lots of work is done on the back-end, while processing a single request, so that the new incoming request doesn't have to wait if the processing is not complete. While processing a request, Node.js attaches a callback function to it and moves it to the back-end. Now, whenever its response is ready, an event is called which triggers the associated callback function to send this response.

Intermediate - What is a callback function?

A function written in a client script that runs asynchronously.

What is a REST collection?

A group of elements with the same representation /posts /users /tasks

Junior - What do you mean by Asynchronous API?

All APIs of Node.js library are aynchronous that is non-blocking. It essentially means a Node.js based server never waits for a API to return data. Server moves to next API after calling it and a notification mechanism of Events of Node.js helps server to get response from the previous API call.

Intermediate - What is Event Emmitter?

All objects that emit events are members of EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object. When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. const EventEmitter = require('events'); class MyEmitter extends EventEmitter {} const myEmitter = new MyEmitter(); myEmitter.on('event', () => { console.log('an event occurred!'); }); myEmitter.emit('event');

What is REST

An architectural style for building APIs. (Stands for 'Representational State Transfer'. We decide that HTTP verbs and URLs mean something.

What is an API

An interface that hides details of an implementation.

Junior - What are the benefits of using Node.js?

Aynchronous and Event DrivenAll APIs of Node.js library are aynchronous that is non-blocking. It essentially means a Node.js based server never waits for a API to return data. Server moves to next API after calling it and a notification mechanism of Events of Node.js helps server to get response from the previous API call. Very Fast Being built on Google Chrome's V8 JavaScript Engine, Node.js library is very fast in code execution. Single Threaded but highly Scalable - Node.js uses a single threaded model with event looping. Event mechanism helps server to respond in a non-bloking ways and makes server highly scalable as opposed to traditional servers which create limited threads to handle requests. Node.js uses a single threaded program and same program can services much larger number of requests than traditional server like Apache HTTP Server. No Buffering - Node.js applications never buffer any data. These applications simply output the data in chunks.

Intermediate - What is purpose of Buffer class in Node?

Buffer class is a global class and can be accessed in application without importing buffer module. A Buffer is a kind of an array of integers and corresponds to a raw memory allocation outside the V8 heap. A Buffer cannot be resized.

Intermediate - How Node prevents blocking code?

By providing callback function. Callback function gets called whenever corresponding event triggered.

When to not use Node.js?

CPU intensive operations

Intermediate - What is Chaining in Node?

Chanining is a mechanism to connect output of one stream to another stream and create a chain of multiple stream operations. It is normally used with piping operations.

Name some of the events fired by streams.

Each type of Stream is an EventEmitter instance and throws several events at different instance of times. For example, some of the commonly used events are: data - This event is fired when there is data is available to read. end - This event is fired when there is no more data to read. error - This event is fired when there is any error receiving or writing data. finish - This event is fired when all data has been flushed to underlying system

Explain some Error Handling approaches in Node.js you know about. Which one will you use?

Error return value - doesn't work asynchronously Error throwing - well-establish pattern, in which a function does its thing and if an error situation arises, it simply bails out throwing an error. Can leave you in an unstable state. It requires extra work to catch them. Also wrapping the async calls in try/catch won't help because the errors happen asynchronously. To fix this, we need domains. Domains provide an asynchronous try...catch. var validateObject = function (obj) { if (typeof obj !== 'object') { throw new Error('Invalid object'); } }; try { validateObject('123'); } catch (err) { console.log('Thrown: ' + err.message); Error callback - returning an error via a callback is the most common error handling pattern in Node.js. Handling error callbacks can become a mess (callback hell or the pyramid of doom). var validateObject = function (obj, callback) { if (typeof obj !== 'object') { return callback(new Error('Invalid object')); } return callback(); }; validateObject('123', function (err) { console.log('Callback: ' + err.message); }); Error emitting - when emitting errors, the errors are broadcast to any interested subscribers and handled within the same process tick, in the order subscribed. var Events = require('events'); var emitter = new Events.EventEmitter(); var validateObject = function (a) { if (typeof a !== 'object') { emitter.emit('error', new Error('Invalid object')); } }; emitter.on('error', function (err) { console.log('Emitted: ' + err.message); }); validateObject('123'); Promises for async error handling Try...catch with async/await - ES7 Async/await allows us as developers to write asynchronous JS code that look synchronous.

Junior - What are event listeners?

Event Listeners are similar to call back functions but are associated with some event. For example when a server listens to http request on a given port a event will be generated and to specify http server has received and will invoke corresponding event listener. Basically, Event listener's are also call backs for a corresponding event. Node.js has built in event's and built in event listeners. Node.js also provides functionality to create Custom events and Custom Event listeners.

Intermediate - What is difference between synchronous and asynchronous method of fs module?

Every method in fs module has synchronous as well as asynchronous form. Asynchronous methods takes a last parameter as completion function callback and first parameter of the callback function is error. It is preferred to use asynchronous method instead of synchronous method as former never block the program execution where the latter one does.

What are the endpoints for a REST element?

GET - Get a single element GET /tasks/123 PUT - Replace an element PUT /tasks/123 DELETE - Delete an element DELETE /tasks/123

What are the endpoints for a REST collection?

GET - List collection elements POST - Insert an element to the collection PUT - Replace the entire element list DELETE - Delete the element list

Intermediate - What is a blocking code?

If application has to wait for some I/O operation in order to complete its execution any further then the code responsible for waiting is known as blocking code.

What is the difference between ./ and __dirname?

In Node.js, __dirname is always the directory in which the currently executing script resides (see this). So if you typed __dirname into /d1/d2/myscript.js, the value would be /d1/d2.

Junior - What is a control flow function?

It is a generic piece of code which runs in between several asynchronous function calls is known as control flow function. Allows for, if, try/catch following await

What tools can be used to assure consistent code style?

JSLint, ESLint

Why should you separate Express 'app' and 'server'?

Keeping the API declaration separated from the network related configuration (port, protocol, etc) allows testing the API in-process, without performing network calls, with all the benefits that it brings to the table: fast testing execution and getting coverage metrics of the code. It also allows deploying the same API under flexible and different network conditions. Bonus: better separation of concerns and cleaner code.

Explain usage of NODE_ENV

Node encourages the convention of using a variable called NODE_ENV to flag whether we're in production right now. . This determination allows components to provide better diagnostics during development, for example by disabling caching or emitting verbose log statements. Setting NODE_ENV to production makes your application 3 times faster.

Junior - If Node.js is single threaded then how it handles concurrency?

Node provides a single thread to programmers so that code can be written easily and without bottleneck. Node internally uses multiple POSIX threads for various I/O operations such as File, DNS, Network calls etc. When Node gets I/O request it creates or uses a thread to perform that I/O operation and once the operation is done, it pushes the result to the event queue. On each such event, event loop runs and checks the queue and if the execution stack of Node is empty then it adds the queue result to execution stack. This is how Node manages concurrency.

Intermediate - How does Node.js handle child threads?

Node.js, in its essence, is a single thread process. It does not expose child threads and thread management methods to the developer. Technically, Node.js does spawn child threads for certain tasks such as asynchronous I/O, but these run behind the scenes and do not execute any application JavaScript code, nor block the main event loop. If threading support is desired in a Node.js application, there are tools available to enable it, such as the ChildProcess module.

What is an API used for?

Perform tasks. Retrieving data. Manipulating data.

What is Piping in Node?

Piping is a mechanism to connect output of one stream to another stream. It is normally used to get data from one stream and to pass output of that stream to another stream. There is no limit on piping operations.

Why to use Buffers instead of binary strings to handle binary data ?

Pure JavaScript does not able to handle straight binary data very well. Since Node.js servers have to deal with TCP streams for reading and writing of data, binary strings will become problematic to work with as it is very slow and has a tendency to break. That's why it is always advisable to use Buffers instead of binary strings to handle binary data.

Intermediate - What is REPL in context of Node?

REPL stands for Read Eval Print Loop and it represents a computer environment like a window console or unix/linux shell where a command is entered and system responds with an output. Node.js or Node comes bundled with a REPL environment. It performs the following desired tasks. Read - Reads user's input, parse the input into JavaScript data-structure and stores in memory. Eval - Takes and evaluates the data structure Print - Prints the result Loop - Loops the above command until user press ctrl-c twice.

What are the rules of REST architecture?

REST servers are stateless. REST uses client to handle UI concerns and server to handle data storage concerns. REST responses must be labeled as cacheable or not. REST should use uniform interface, it all resources are accessed using HTTP. Resource endpoints do NOT need to mimic the data on the server. IE, /agenda may access data from 'events' and 'tasks' REST should uses a layered system. Components of an API can transform content of messages or utilize middleware to check security for example.

Explain what is Reactor Pattern in Node.js?

Reactor Pattern is an idea of non-blocking I/O operations in Node.js. This pattern provides a handler(in case of Node.js, a callback function) that is associated with each I/O operation. When an I/O request is generated, it is submitted to a demultiplexer.

What does REST stand for?

Representations State Transfer

How can you listen on port 80 with Node?

Run the application on any port above 1024, then put a reverse proxy like nginx (http://nginx.org/) in front of it.

Junior - Why scalable with Node.js?

Scalability is baked into Node and child processes are the key factors to scale our application. You can use child process to run system commands, read large files without blocking event loop, decompose the application into various "nodes" (That's why it's called Node).

Intermediate - What is stream and what are types of streams available in Node.js?

Streams are a collection of data that might not be available all at once and don't have to fit in memory. Streams provide chunks of data in a continuous manner. It is useful to read a large set of data and process it. There is four fundamental type of streams: Readable. Writeable. Duplex. Transform. Readable streams as the name suggest used in reading a large chunk of data from a source. Writable streams are used in writing a large chunk of data to the destination. Duplex streams are both readable and writable ( Eg socket). Transform stream is the duplex stream which is used in modifying the data (eg zip creation).

What's a stub? Name a use case

Stubs are functions/programs that simulate the behaviours of components/modules. Stubs provide canned answers to function calls made during test cases. Also, you can assert on with what these stubs were called. A use-case can be a file read, when you do not want to read an actual file:

What is the purpose of __dirname variable?

The __dirname represents the name of the directory that the currently executing script resides in.

What is the purpose of __filename variable?

The __filename represents the filename of the code being executed. This is the resolved absolute path of this code file. For a main program this is not necessarily the same filename used in the command line. The value inside a module is the path to that module file.

Intermediate - What's the event loop?

The event loop is what allows Node.js to perform non-blocking I/O operations — despite the fact that JavaScript is single-threaded — by offloading operations to the system kernel whenever possible. Every I/O requires a callback - once they are done they are pushed onto the event loop for execution. Since most modern kernels are multi-threaded, they can handle multiple operations executing in the background. When one of these operations completes, the kernel tells Node.js so that the appropriate callback may be added to the poll queue to eventually be executed.

Intermediate - What is the purpose of setTimeout function?

The setTimeout(cb, ms) global function is used to run callback cb after at least ms milliseconds. The actual delay depends on external factors like OS timer granularity and system load. A timer cannot span more than 24.8 days.

Intermediate - What are the global objects of Node.js?

These objects are available in all modules: process - The process object is a global that provides information about, and control over, the current Node.js process. console - Used to print to stdout and stderr. buffer - Used to handle binary data.

How to solve "Process out of Memory Exception" in Node.js

To solve the process out of memory exception in Node.js we need to increase the max-oldspace-size. By default the max size of max-old-space-size is 512 mb which you can increase by the command: node --max-old-space-size=1024 file.js

Intermediate - What is the preferred method of resolving unhandled exceptions in Node.js?

Unhandled exceptions in Node.js can be caught at the Process level by attaching a handler for uncaughtException event. process.on('uncaughtException', function(err) { console.log('Caught exception: ' + err); }); However, uncaughtException is a very crude mechanism for exception handling and may be removed from Node.js in the future. An exception that has bubbled all the way up to the Process level means that your application, and Node.js may be in an undefined state, and the only sensible approach would be to restart everything. The preferred way is to add another layer between your application and the Node.js process which is called the domain. Domains provide a way to handle multiple different I/O operations as a single group. So, by having your application, or part of it, running in a separate domain, you can safely handle exceptions at the domain level, before they reach the Process level.

Junior - How to debug in Node

Using the native Node debugger, you can set a breakpoint using 'debugger' and step into and over call stack and event loop. Using the node inspector, you can connect to a node process using Chrome dev tools. Pass --inspect when running node.

How to gracefully Shutdown Node.js Server?

We can gracefully shutdown Node.js server by using the generic signal called SIGTERM or SIGINT which is used for program termination. We need to call SIGTERM or SIGINT which will terminate the program and clean up the resources utilized by the program.

Junior - How you can monitor a file for modifications in Node.js ?

We can take advantage of File System watch() function which watches the changes of the file.

Intermediate - When should I use EventEmitter?

Whenever it makes sense for code to subscribe to something rather than get a callback from something. The typical use case would be that there's multiple blocks of code in your application that may need to do something when an event happens.

Consider following code snippet: { console.time("loop"); for (var i = 0; i < 1000000; i += 1) { // Do nothing } console.timeEnd("loop"); } The time required to run this code in Google Chrome is considerably more than the time required to run it in Node.js Explain why this is so, even though both use the v8 JavaScript Engine.

Within a web browser such as Chrome, declaring the variable i outside of any function's scope makes it global and therefore binds it as a property of the window object. As a result, running this code in a web browser requires repeatedly resolving the property i within the heavily populated window namespace in each iteration of the for loop. In Node.js, however, declaring any variable outside of any function's scope binds it only to the module's own scope (not the window object) which therefore makes it much easier and faster to resolve.

Junior - Is Node.js single threaded?

Yes! Node uses a single threaded model with event looping.

Does Node.js support multi-core platforms? And is it capable of utilizing all the cores?

Yes, Node.js would run on a multi-core system without any issue. But it is by default a singlethreaded application, so it can't completely utilize the multi-core system. However, Node.js can facilitate deployment on multi-core systems where it does use the additional hardware. It packages with a Cluster module which is capable of starting multiple Node.js worker processes that will share the same port.

Is Node.js entirely based on a single-thread?

Yes, it's true that Node.js processes all requests on a single thread. But it's just a part of the theory behind Node.js design. In fact, more than the single thread mechanism, it makes use of events and callbacks to handle a large no. of requests asynchronously. Moreover, Node.js has an optimized design which utilizes both JavaScript and C++ to guarantee maximum performance. JavaScript executes at the server-side by Google Chrome v8 engine. And the C++ lib UV library takes care of the non-sequential I/O via background workers. To explain it practically, let's assume there are 100s of requests lined up in Node.js queue. As per design, the main thread of Node.js event loop will receive all of them and forwards to background workers for execution. Once the workers finish processing requests, the registered callbacks get notified on event loop thread to pass the result back to the user.

How do you catch an error from await?

await someFunction().catch((error)=>console.log(error));

What is the difference between http-request and http-response

http-request - sent when requesting a network based resource. Contains: - URL path - Method - Headers - Body http-response - received from a http-request Contains: - Status code (200 - OK, 404 - Not found, 301 - Redirect, 500 - Error) - Headers - Body

How would you handle errors for async code in Node.js?

it is better to use a reputable promise library or async-await instead which enables a much more compact and familiar code syntax like try-catch.

What is the difference between res.send and res.end?

res.send - Properly formats a response header and other headers along with content. res.end - Does not include content header

What are the timing features of Node.js?

setTimeout/clearTimeout - can be used to schedule code execution after a designated amount of milliseconds setInterval/clearInterval - can be used to execute a block of code multiple times setImmediate/clearImmediate - will execute code at the end of the current event loop cycle process.nextTick - used to schedule a callback function to be invoked in the next iteration of the Event Loop

Tell 3 ways to create child processes -

spawn - child_process.spawn launches a new process with a given command. exec - child_process.exec method runs a command in a shell/console and buffers the output. fork - The child_process.fork method is a special case of the spawn() to create child processes.

Junior - How to make Post request in Node.js?

var request = require('request'); request.post('http://www.example.com/action', { form: { key: 'value' } }, function(error, response, body) { if (!error && response.statusCode == 200) { console.log(body) } });