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How to avoid callback hell?
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Section 1
(50 cards)
How to avoid callback hell?
* modularization -> break callbacks into independent functions * use async/await * use generators with Promises
Which is the first argument usually passed to a Node.js callback handler?
error object (may be null if no errors occur)
What is event emitter?
EventEmitter lies in events module let events = require('events'); let eventEmitter = new events.EventEmitter(); EventEmitter provides properties like on and emit - on is used to bind a function with an event - emit is used to fire an event
What are streams in Node.js?
Streams are objects that allow reading of data from the source and writing of data to the destination as a continuous process
Explain the event loop?
See https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop * TL;DR allows javascript to perform non-blocking I/O operations
What keyword do you need to use to insert a debug breakpoint?
debugger keyword
What tools can be used to assure consistent style? Why is this important?
ESLint... it is important so team members can modify more projects easily and understand how code is written without having to relearn different styles
When are background/worker processes useful?
These are useful when you want to do data processing in the background such as sending out emails or processing images
Name the types of API functions in Node.js
* Blocking functions -> all other code is blocked from executing until I/O event that is being waited on occurs *Non-Blocking Functions: Multiple I/O calls can be performed without the execution of the program being halted
What is a prototype chain?
object -> reference to object -> reference to object person1 instance prototype inherits from Person prototype which inherits from Object prototype
How to read a file using Node?
fs.read(fd, buffer, offset, length, position, callback) fd -> file descriptor returned by fs.open() method buffer -> this is the buffer the data will be written to offset -> offset in buffer to start writing at length -> integer specifying number of bytes to read position -> an integer specifying where to begin reading from the file callback -> call back function that gets params (err, bytesRead, buffer)
Object literal vs Object instantiation?
1. Object literal let obj1 = {} has no constructors 2. Object instantiation using new keyword to create objects and have constructor 3. object literal variable acts as one instance (singleton) ... setting variables equal to them refer to the same instance! 4. Object instantiation approach allows you to make different objects of the same type
What are exit codes in Node.js?
Exit codes are specific codes used to end a "process 1. Unused 2. Uncaught Fatal Exception 3. Fatal Error 4. Non-function Internal Exception Handler 5. Internal Exception handler Run-Time Failure 6. Internal JavaScript Evaluation Failure
What is Buffer class in Node?
Buffer class is a global class that can be accessed in the app without importing buffer module. A buffer is kind of array of ints and corresponds to raw memory allocation outside the V8 heap
What is __dirname?
A global variable that returns the absolute path of directory containing the current file
What is Node.js?
Node.js is a web app framework built on Google Chrome's JavaScript V8 engine. Runtime allows to execute js code on any machine outside a web browser.
What are 2 methods in the fs module that can be used for reading a whole file at once?
fs.readFile and fs.readFileSync
What is a blocking function
Must be completed before other statements are executed
Explain chaining in Node.js?
Chaining is a mechanism whereby the output of 1 stream is connected to another stream creating a chain of multiple stream operations
What is Node.js?
Is a server side scripting based on Google's V8 JavaScript engine. Used to build web apps
Name Truthy Values in JS
1. true keyword 2. empty object {} 3. empty array [] 4. non-zero number 42 or -42 5. new Date() 6. 12n bigint 7. 3.14 or -3.14 floats 8. Infinity or -Infinity
Name some flags used to read/write operation on files
r -Open file for reading. exception occurs if file does not exist r+ -> open for file read and write rs -> open file for reading in sync mode w -> open file for writing. creates file if it doesn't exist or overwrites file if it exists a -> open file for appending
What is a callback?
Callback is an asynchronous equivalent for a function. Callbacks are called when a task is completed.
How are objects that generate events called and which class from the events module are they instances of?
Objects are called event emitters and are instances of EventEmitter class.
What are Globals in Node.js?
* Global - represents Global namespace object and acts as a container for all other <global> objects * Process - one of the global objects but can turn a synchronous function into an async callback. It can be accessed anywhere in the code and it primarily gives back info about app or environment * Buffer: it is a class in Node.js to handle binary data
What is a callback function?
A function written in a client script that runs asynchronously. Can finish at anytime depending if there is network call to be made internally
What are async and await?
The await expression pauses execution of function and waits for resolution of passed promise. Then function's execution resumes and returns resolved value. (Makes code run synchronously easier to think about)
Difference between Node.js and Ajax?
Ajax (Async Javascript and XML) is designed for dynamically updating a particular section of a page's content without having to update the entire page. Node.js used for developing client-server applications
What is Piping in Node?
Piping is a mechanism to connect output of onstream to another stream. Used to get data from one stream and output to another stream.
Which global variable can be used to access information about the app and the environment that it runs in?
The process variable
What is LTS releases of Node.js why should I care?
LTS = (Long Term Support) version of Node.js receive critical bug fixes, security updates and performance udpates * LTS versions are supported for at least 18 months * Best for production since LTS release line is focused on stability and security whereas Current release line has a shorter lifespan and more frequent updates to the code
Name Falsy values in JS
1. false keyword 2. number 0 3. BigInt 0n 4. "", '', or ``, empty strings 5. null 6. undefined 7. NaN
T/F Node.js is multi-threaded
False
What is Node.js Cluster module?
module used to take advantage of multi-core systems, so apps can handle more load
What is REPL?
Read Evaluate Print Loop
Which keywords are used to export and import an object or a function from a module file?
module.exports to export an object or function require to import an object or function
What's a stub?
Stubs are functions/programs that simulate behaviors of components/modules. Stubs provide hypothetical answers to function calls made during test cases.
What's the difference between setTimeout and setImmediate?
setTimeout is used to run a function after a certain minimal timeout. setImmediate is used to run a function once the current event loop completes.
Why is JavaScript described as a prototype-based language?
Objects in JavaScript are considered template objects where they inherit methods and properties from.
What is callback hell?
Results in heavily nested callbacks that make code not only unreadable but difficult to maintain
Name the 4 types of streams
* <Readable> to facilitate the reading operation * <Writable> to facilitate the writing operation * <Duplex> to facilitate both read and write operations * <Transform> is a form of Duplex stream that performs computations based on the available input
What is the preferred method of resolving unhandled exceptions in node.js
exceptions can be caught at the process level. process.on('uncaughtException', (err) => { console.log(`caught exception: ${err}`); });
What are the functionalities of NPM in Node.js?
Node Package Manager: 1. Online repo for Node.js packages 2. Command Line utility for installing packages, version management, and dependency management of Node.js packages
What is libuv?
C library that implements the event loop and all of async behaviors
What does "callback hell" mean?
Plenty of nested callbacks making code difficult to read and maintain
How can you secure your HTTP cookies against XSS attacks?
* XSS (Cross Site Scripting) occurs when the attacker injects executable JS code into HTML response. * To mitigate the attack, must set flags on the set-cookie HTTP header: HttpOnly - attribute used to help prevent attacks such as xss since it does not allow cookie to be accessed via Javascript
What are promises?
* A Promise can either be resolved or rejected * Useful for making async operations * const promise1 = new Promise(function(resolve, reject) { setTimeout(function() { resolve('foo'); }, 300); });
T/F Prototype chains copy methods and functions from one object to another
False. They are accessed by walking up the chain if child prototype does not contain a function definition for it
How many types of streams are present in Node?
4 types of streams are present. 1. Readable - Stream which is used for read operation 2. Writable - Stream which is used for write operation 3. Duplex - stream for both read and write operation 4. Transform -> type of duplex stream where output is computed based on input
How to open a file using Node?
fs.open(path, flags[, mode], callback) path -. string containing path to file flags -> tells behavior of file to be opened ('w' as example) mode -> sets the file mode (permission and sticky bits) callback -> function which gets 2 arguments (err, fd)
Section 2
(3 cards)