What does async false do in jQuery ajax

Question

Specifically  how does it differ from the default    async  true      In what circumstances would I want to explicit set async to false  and does it have something to do with preventing other events on the page from firing

User · Answer

If you disable asynchronous retrieval  your script will block until the request has been fulfilled  It s useful for performing some sequence of requests in a known order  though I find async callbacks to be cleaner

User · Answer

async false   Code paused   Other code waiting for this to finish   async true   Code continued   Nothing gets paused  Other code is not waiting     As simple as this

User · Answer

Setting async to false means the instructions following the ajax request will have to wait for the request to complete  Below is one case where one have to set async to false  for the code to work properly   var phpData    function get php data       var php data      ajax       url   http   somesite v1 api get php data       async  false         very important  else php data will be returned even before we get Json from the url     dataType   json       success  function  json          php data   json                return php data          Above example clearly explains the usage of async false  By setting it to false  we have made sure that once the data is retreived from the url  only after that return php data  is called

User · Answer

Does it have something to do with   preventing other events on the page   from firing    Yes   Setting async to false means that the statement you are calling has to complete before the next statement in your function can be called   If you set async  true then that statement will begin it s execution and the next statement will be called regardless of whether the async statement has completed yet   For more insight see   jQuery ajax success anonymous function scope

User · Answer

From  https   xhr spec whatwg org  synchronous-flag  Synchronous XMLHttpRequest outside of workers is in the process of being removed from the web platform as it has detrimental effects to the end user s experience   This is a long process that takes many years   Developers must not pass false for the async argument when the JavaScript global environment is a document environment  User agents are strongly encouraged to warn about such usage in developer tools and may experiment with throwing an InvalidAccessError exception when it occurs  The future direction is to only allow XMLHttpRequests in worker threads  The message is intended to be a warning to that effect

User · Answer

One use case is to make an ajax call before the user closes the window or leaves the page   This would be like deleting some temporary records in the database before the user can navigate to another site or closes the browser      window  unload          function                  ajax               url   your url               global  false              type   POST               data                  async  false    blocks window close             success  function

User · Answer

Async False will hold the execution of rest code  Once you get response of ajax  only then  rest of the code will execute

[jquery] What does "async: false" do in jQuery.ajax()?

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