Introduction to Node JS2.pdf
- 1. Prepared By: Prof. Bareen Shaikh Department of Computer Science, MIT ACSC,ALNDI(D), PUNE Introduction to Nodejs
- 2. Synchronous & Asynchronous Synchronous Programming Waits for each statement to complete its execution. Unless and until the current statement gets execute it never execute next statement. It slow downs the application process. Instruction/statements are in queue for execution. Asynchronous Programming It doesn’t wait for current statement execution before moving to next statement. Application process becomes fast. Three approaches makes programming asynchronous Callbacks Promises Async/Await
- 3. Callback in JavaScript In JavaScript Functions are first-class objects in and as such can functions have the ability to: Be assigned to variables (and treated as a value). Have other functions in them.(HOF) Return other functions to be called later. Callback functions A callback is simply a function that is registered as the event handler for some event we know will happen in the future. When a function simply accepts another function as an argument, this contained function is known as a callback function. Callback functions is a core functional programming concept. Eg. setInterval(function() { document.write('hello!'); },3000),
- 4. Callback in JavaScript Multiple functions can be created independently and used as callback functions.(called as callback hell) function (f1(arg1) { f2(arg2) { f3(arg3) { f4(arg4) { f5(arg5) { document. write('done'); //let's begin to close each function! }; }; }; }; })
- 5. Promises in JavaScript Promises were introduced to simplify deferred activities. I promise to do this whenever that is true.If it isn't true,then I won't. This is a simple illustration of JavaScript Promises. A promise is used to handle the asynchronous result of an operation. Promises are the ideal choice for handling asynchronous operations in the simplest manner. They can handle multiple asynchronous operations easily Provide better error handling than callbacks and events. Benefits of Promises Improves Code Readability Better handling of asynchronous operations Better flow of control definition in asynchronous logic Better Error Handling
- 6. Promises in JavaScript A Promise has four states: fulfilled:Action related to the promise succeeded rejected:Action related to the promise failed pending: Promise is still pending i.e not fulfilled or rejected yet settled: Promise has fulfilled or rejected A promise can be created using Promise constructor. Syntax var promise = new Promise(function(resolve, reject){ //do something });
- 8. Promises in JavaScript Promises notify whether the request is fulfilled or rejected. Callbacks can be registered with the .then() to handle fulfillment and rejection. The .then() can be chained to handle the fulfillment and rejection. .then() then() is invoked when a promise is either resolved or rejected. .then() method takes two functions as parameters. Syntax: .then(function(result){ //handle success }, function(error){ //handle error }) Whereas .catch() can be used for handling the errors.
- 9. Promises in JavaScript catch() catch() is invoked when a promise is either rejected or some error has occurred in execution. catch() method takes one function as parameter. Function to handle errors or promise rejections. .catch() is just a shorthand for .then(null, errorHandler) Syntax: .catch(function(error){ //handle error })
- 10. Example of Promises in JavaScript var promise = new Promise(function(resolve, reject) { const x = 25; const y = 25; if(x === y) { resolve(); } else { reject(); } }); promise. then(function () { document.write(‘Equal'); }). catch(function () { document.write(‘Not Equal'); });
- 11. Callback & Promises in JavaScript
- 12. NodeJS Event loop Node.js is a single-threaded event-driven platform that is capable of running non-blocking, asynchronously programming. Functionalities of Node.js make it memory efficient. Event loop allows Node.js to perform non-blocking I/O operations despite the fact that JavaScript is single-threaded. Achieved by assigning operations to the operating system whenever and wherever possible. Features of Event Loop: Event loop is an endless loop, which waits for tasks, executes them and then sleeps until it receives more tasks. The event loop executes tasks from the event queue only when the call stack is empty i.e. there is no ongoing task. The event loop allows us to use callbacks and promises. The event loop executes the tasks starting from the oldest first.
- 13. Working of NodeJS Event loop
- 14. Phases of NodeJS Event loop
- 15. Phases of NodeJS Event loop timers: this phase executes callbacks scheduled by setTimeout() and setInterval(). pending callbacks: executes I/O callbacks deferred to the next loop iteration. idle, prepare: only used internally. poll: retrieve new I/O events; execute I/O related callbacks (almost all with the exception of close callbacks, the ones scheduled by timers, and setImmediate()); node will block here when appropriate. check: setImmediate() callbacks are invoked here. close callbacks: some close callbacks, e.g. socket.on('close', ...).
- 16. Thank You