What is the difference between synchronous and asynchronous programming in node js

Question

I ve been reading nodebeginner And I came across the following two pieces of code   The first one       var result   database query  SELECT   FROM hugetable        console log  Hello World      The second one       database query  SELECT   FROM hugetable   function rows           var result   rows              console log  Hello World      I get what they re supposed to do  they query the database to retrieve the answer to the query  And then console log  Hello world     The first one is supposedly synchronous code   And the second one is asynchronous code   The difference between the two pieces is very vague to me  What would the output be   Googling on asynchronous programming didn t help me either

User · Answer

Synchronous functions are blocking while asynchronous functions are not. In synchronous functions, statements complete before the next statement is run. In this case, the program is evaluated exactly in order of the statements and execution of the program is paused if one of the statements take a very long time.

Asynchronous functions usually accept a callback as a parameter and execution continue on the next line immediately after the asynchronous function is invoked. The callback is only invoked when the asynchronous operation is complete and the call stack is empty. Heavy duty operations such as loading data from a web server or querying a database should be done asynchronously so that the main thread can continue executing other operations instead of blocking until that long operation to complete (in the case of browsers, the UI will freeze).

Orginal Posted on Github: Link

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This would become a bit more clear if you add a line to both examples   var result   database query  SELECT   FROM hugetable    console log result length   console log  Hello World      The second one   database query  SELECT   FROM hugetable   function rows       var result   rows     console log result length       console log  Hello World      Try running these  and you   ll notice that the first  synchronous  example  the result length will be printed out BEFORE the  Hello World  line  In the second  the asynchronous  example  the result length will  most likely  be printed AFTER the  Hello World  line   That s because in the second example  the database query is run asynchronously in the background  and the script continues straightaway with the  Hello World   The console log result length  is only executed when the database query has completed

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Asynchronous programming in JS   Synchronous    Stops execution of further code until this is done  Because it this stoppage of further execution  synchronous code is called  blocking   Blocking in the sense that no other code will be executed    Asynchronous   Execution of this is deferred to the event loop  this is a construct in a JS virtual machine which executes asynchronous functions  after the stack of synchronous functions is empty   Asynchronous code is called non blocking because it doesn t block further code from running    Example    x000D   x000D     This function is synchronous x000D  function log arg    x000D      console log arg  x000D    x000D   x000D  log 1   x000D   x000D     This function is asynchronous x000D  setTimeout      gt    x000D      console log 2  x000D     0   x000D   x000D  log 3  x000D   x000D   x000D     The example logs 1  3  2  2 is logged last because it is inside a asynchronous function which is executed after the stack is empty

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First  I realize I am late in answering this question   Before discussing synchronous and asynchronous  let us briefly look at how programs run   In the synchronous case  each statement completes before the next statement is run  In this case the program is evaluated exactly in order of the statements   This is how asynchronous works in JavaScript  There are two parts in the JavaScript engine  one part that looks at the code and enqueues operations and another that processes the queue  The queue processing happens in one thread  that is why only one operation can happen at a time   When an asynchronous operation  like the second database query  is seen  the code is parsed and the operation is put in the queue  but in this case a callback is registered to be run when this operation completes  The queue may have many operations in it already  The operation at the front of the queue is processed and removed from the queue  Once the operation for the database query is processed  the request is sent to the database and when complete the callback will be executed on completion  At this time  the queue processor having  handled  the operation moves on the next operation - in this case       console log  Hello World       The database query is still being processed  but the console log operation is at the front of the queue and gets processed  This being a synchronous operation gets executed right away resulting immediately in the output  Hello World   Some time later  the database operation completes  only then the callback registered with the query is called and processed  setting the value of the variable result to rows   It is possible that one asynchronous operation will result in another asynchronous operation  this second operation will be put in the queue and when it comes to the front of the queue it will be processed  Calling the callback registered with an asynchronous operation is how JavaScript run time returns the outcome of the operation when it is done    A simple method of knowing which JavaScript operation is asynchronous is to note if it requires a callback - the callback is the code that will get executed when the first operation is complete  In the two examples in the question  we can see only the second case has a callback  so it is the asynchronous operation of the two  It is not always the case because of the different styles of handling the outcome of an asynchronous operation    To learn more  read about promises  Promises are another way in which the outcome of an asynchronous operation can be handled  The nice thing about promises is that the coding style feels more like synchronous code    Many libraries like node  fs   provide both synchronous and asynchronous styles for some operations  In cases where the operation does not take long and is not used a lot - as in the case of reading a config file - the synchronous style operation will result in code that is easier to read

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The difference is that in the first example  the program will block in the first line  The next line  console log  will have to wait   In the second example  the console log will be executed WHILE the query is being processed  That is  the query will be processed in the background  while your program is doing other things  and once the query data is ready  you will do whatever you want with it   So  in a nutshell  The first example will block  while the second won t   The output of the following two examples      Example 1 - Synchronous  blocks  var result   database query  SELECT   FROM hugetable    console log  Query finished    console log  Next line         Example 2 - Asynchronous  doesn t block   database query  SELECT   FROM hugetable   function result        console log  Query finished        console log  Next line      Would be    Query finished Next line Next line Query finished   Note While Node itself is single threaded  there are some task that can run in parallel  For example  File System operations occur in a different process     That s why Node can do async operations  one thread is doing file system operations  while the main Node thread keeps executing your javascript code  In an event-driven server like Node  the file system thread notifies the main Node thread of certain events such as completion  failure  or progress  along with any data associated with that event  such as the result of a database query or an error message  and the main Node thread decides what to do with that data    You can read more about this here  How the single threaded non blocking IO model works in Node js

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The main difference is with asynchronous programming  you don t stop execution otherwise  You can continue executing other code while the  request  is being made

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In the synchronous case  the console log command is not executed until the SQL query has finished executing   In the asynchronous case  the console log command will be directly executed  The result of the query will then be stored by the  callback  function sometime afterwards

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The difference between these two approaches is as follows   Synchronous way  It waits for each operation to complete  after that only it executes the next operation  For your query  The console log   command will not be executed until  amp  unless the query has finished executing to get all the result from Database   Asynchronous way  It never waits for each operation to complete  rather it executes all operations in the first GO only  The result of each operation will be handled once the result is available  For your query  The console log   command will be executed soon after the Database Query   method  While the Database query runs in the background and loads the result once it is finished retrieving the data   Use cases   If your operations are not doing very heavy lifting like querying huge data from DB then go ahead with Synchronous way otherwise Asynchronous way  In Asynchronous way you can show some Progress indicator to the user while in background you can continue with your heavy weight works  This is an ideal scenario for GUI apps

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Sync Programming  Programming languages like C  C   Java are sync programming  what so ever you write will be execute in order of your writing   -GET DATA FROM SQL    Suppose fetching data take 500 msec  -PERFORM SOME OTHER FUNCTION    Performing some function other will take 100 msec  but execution of other    task start only when fetching of sql data done  i e some other function    can execute only after first in process job finishes    -TOTAL TIME OF EXECUTION IS ALWAYS GREATER THAN  500   100   processing time   msec   Async  NodeJs comes up with async feature  it s non-blocking in nature  suppose in any I O task which is taking time  fetching  writing  reading   nodejs won t kept idle and wait for the task to be finish  it ll start executing next tasks in the queue  and whenever that time taking task completed it will notify using callback  Following example will help     Nodejs uses callback pattern to describe functions    Please read callback pattern to understand this example    Suppose following function  I O involved  took 500 msec function timeConsumingFunction params  callback       GET DATA FROM SQL   getDataFromSql params  function error  results       if error         callback error             else        callback null  results                   Suppose following function is non-blocking and took 100 msec function someOtherTask        some other task   console log  Some Task 1      console log  Some Task 2       console log  Execution Start       Start With this function timeConsumingFunction params  function error  results       if error         console log  Error             else        console log  Successfull                   As  suppose  timeConsumingFunction took 500 msec     As NodeJs is non-blocking  rather than remain idle for 500 msec  it will start    execute following function immediately someOtherTask      In Short  Output is as   Execution Start   Roughly after 105 msec  5 msec it ll take in processing  Some Task 1 Some Task 2   Roughly After 510 msec Error Successful   depends on success and failure of DB function execution   Difference is clear where sync will definitely take more than 600  500   100   processing time  msec  async saves time

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The function makes the second one asynchronous   The first one forces the program to wait for each line to finish it s run before the next one can continue  The second one allows each line to run together  and independently  at once   Languages and frameworks  js  node js  that allow asynchronous or concurrency is great for things that require real time transmission  eg  chat  stock applications

[javascript] What is the difference between synchronous and asynchronous programming (in node.js)

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