Asynchronous JavaScript and Promises

Asynchronous JavaScript and Promises

• 1.
  Async JavaScript and Promises Sunday, 25August 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? Allof 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
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  An entity thathandles 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, 25August 13
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  Event Loop Sunday, 25August 13
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  Making Code Asynchronous setImmediate process.nextTick setTimeout /setInterval Sunday, 25 August 13
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  Promises A Promise isan 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
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  Handling exceptions Promises alsohelps 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 thecode 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 snippetsused for the demo can be downloaded from https://gist.github.com/senthilkumarv/6326192 Contact at senthilkumar@thoughtworks.com and kuldeepg@thoughtworks.com Sunday, 25 August 13