Asynchronous JavaScript and Promises
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This document discusses asynchronous JavaScript and promises. It explains that JavaScript is single-threaded but asynchronous via an event loop. Promises represent asynchronous events and can help solve issues with callback hell by allowing asynchronous code to be written sequentially. Promises handle errors cleanly and make asynchronous code appear synchronous by adding functions to the event loop to be resolved on the next pass.
Asynchronous JavaScript and Promises
- 1. Async JavaScript and Promises Sunday, 25 August 13
- 2. Callbacks console.log("Before readFile"); fs.readFile("file1.txt", function(file1Content) { console.log("File Contents here"); return "#" + file1Content + "#"; }); console.log("After readFile"); Functions are first class citizens in JavaScript Functions can takes functions as argument and call them when they are done Sunday, 25 August 13
- 3. Callbacks console.log("Before readFile"); fs.readFile("file1.txt", function(file1Content) { console.log("File Contents here"); return "#" + file1Content + "#"; }); console.log("After readFile"); Functions are first class citizens in JavaScript Functions can takes functions as argument and call them when they are done Sunday, 25 August 13
- 4. Is JavaScript really Async? All of the JavaScript engines including V8 are single-threaded But all I/O is evented and asynchronous It is possible via the EventLoop Sunday, 25 August 13
- 5. An entity that handles and processes external events and converts them into callback invocations Every asynchronous operation adds itself to the EventLoop Event Loops Sunday, 25 August 13
- 6. How EventLoop works? Sunday, 25 August 13
- 7. Event Loop Sunday, 25 August 13
- 8. Making Code Asynchronous setImmediate process.nextTick setTimeout / setInterval Sunday, 25 August 13
- 9. Promises A Promise is an object that represents a one-time event, typically the outcome of an async task like an AJAX call. At first, a Promise is in a pending state. Eventually, it’s either resolved or rejected. Once a Promise is resolved or rejected, it’ll remain in that state forever, and its callbacks will never fire again. Sunday, 25 August 13
- 10. What are Promises For? Sunday, 25 August 13
- 11. Callback Hell step1(function (value1) { step2(value1, function(value2) { step3(value2, function(value3) { step4(value3, function(value4) { // Do something with value4 }); }); }); }); Q.fcall(promisedStep1) .then(promisedStep2) .then(promisedStep3) .then(promisedStep4) .then(function (value4) { // Do something with value4 }) .done(); Promises can solve the problem of “Callback Hell” Sunday, 25 August 13
- 12. Handling exceptions Promises also helps you handle exceptions in a cleaner way var handleError = console.log; step1(function (value1) { step2(value1, function(value2) { step3(value2, function(value3) { step4(value3, function(value4) { // Do something with value4 }, handleError); }, handleError); }, handleError); }, handleError); Q.fcall(promisedStep1) .then(promisedStep2) .then(promisedStep3) .then(promisedStep4) .then(function(value4) { // Do something with value4 }) .fail(function(error) { // Handle any error from all above steps }) .done(); Sunday, 25 August 13
- 13. It makes the code Async Q adds the functions in the chain to the Event loop When the promised function returns, the promises are rejected/resolved during the next pass of the runloop Sunday, 25 August 13
- 14. Thank you Code snippets used for the demo can be downloaded from https://gist.github.com/senthilkumarv/6326192 Contact at [email protected] and [email protected] Sunday, 25 August 13