Why is setState in reactjs Async instead of Sync

Question

I have just found that in react this setState   function in any component is asynchronous or is called after the completion of the function that it was called in   Now I searched and found this blog  setState   State Mutation Operation May Be Synchronous In ReactJS   Here he found that setState is async called when stack is empty  or sync called as soon as called  depending on how the change of state was triggered   Now these two things are hard to digest    In the blog the setState function is called inside a function updateState  but what triggered the updateState function is not something that a called function would know about  Why would they make setState async as JS is single threaded language and this setState is not a WebAPI or server call so has to be done on JS s thread only  Are they doing this so that Re-Rendering does not stop all the event listeners and stuff  or there is some other design issue

User · Answer

1  setState actions are asynchronous and are batched for performance gains  This is explained in the documentation of setState      setState   does not immediately mutate this state but creates a pending state transition  Accessing this state after calling this method can potentially return the existing value    There is no guarantee of synchronous operation of calls to setState and calls may be batched for performance gains     2  Why would they make setState async as JS is a single threaded language and this setState is not a WebAPI or server call   This is because setState alters the state and causes rerendering  This can be an expensive operation and making it synchronous might leave the browser unresponsive   Thus the setState calls are asynchronous as well as batched for better UI experience and performance

User · Answer

Good article here  https   github com vasanthk react-bits blob master patterns 27 passing-function-to-setState md     assuming this state count     0 this setState  count  this state count   1    this setState  count  this state count   1    this setState  count  this state count   1       this state count     1  not 3  Solution this setState  prevState  props    gt       count  prevState count   props increment        or pass callback  this setState          callback   https   medium com javascript-scene setstate-gate-abc10a9b2d82  https   medium freecodecamp org functional-setstate-is-the-future-of-react-374f30401b6b

User · Answer

Yes  setState   is asynchronous   From the link  https   reactjs org docs react-component html setstate        React does not guarantee that the state changes are applied immediately      setState   does not always immediately update the component      Think of setState   as a request rather than an immediate command to update the component       Because they think From the link  https   github com facebook react issues 11527 issuecomment-360199710         we agree that setState   re-rendering synchronously would be inefficient in many cases   Asynchronous setState   makes life very difficult for those getting started and even experienced unfortunately  - unexpected rendering issues  delayed rendering or no rendering  based on program logic  - passing parameters is a big deal among other issues   Below example helped      call doMyTask1 - here we set state    then after state is updated           call to doMyTask2 to proceed further in program  constructor props                      This binding is necessary to make  this  work in the callback     this doMyTask1   this doMyTask1 bind this       this doMyTask2   this doMyTask2 bind this      function doMyTask1 myparam1                   this setState                        mystate1   myvalue1               mystate2   myvalue2                                                 gt                this doMyTask2 myparam1                        function doMyTask2 myparam2                  Hope that helps

User · Answer

setState is asynchronous  You can see in this documentation by Reactjs  https   reactjs org docs faq-state html why-is-setstate-giving-me-the-wrong-valuejs https   reactjs org docs faq-state html when-is-setstate-asynchronous   React intentionally    waits    until all components call setState   in their event handlers before starting to re-render  This boosts performance by avoiding unnecessary re-renders    However  you might still be wondering why React doesn   t just update this state immediately without re-rendering    The reason is this would break the consistency between props and state  causing issues that are very hard to debug   You can still perform functions if it is dependent on the change of the state value  Option 1  Using callback function with setState this setState      value  newValue       gt         It is an callback function        Here you can access the update value    console log this state value      Option 2  using componentDidUpdate This function will be called whenever the state of that particular class changes  componentDidUpdate prevProps  prevState         Here you can check if value of your desired variable is same or not      if this state value     prevState value              this part will execute if your desired variable updates

User · Answer

I know this question is old  but it has been causing a lot of confusion for many reactjs users for a long time  including me  Recently Dan Abramov  from the react team  just wrote up a great explanation as to why the nature of setState is async   https   github com facebook react issues 11527 issuecomment-360199710  setState is meant to be asynchronous  and there are a few really good reasons for that in the linked explanation by Dan Abramov  This doesn t mean it will always be asynchronous - it mainly means that you just can t depend on it being synchronous  ReactJS takes into consideration many variables in the scenario that you re changing the state in  to decide when the state should actually be updated and your component rerendered  A simple example to demonstrate this  is that if you call setState as a reaction to a user action  then the state will probably be updated immediately  although  again  you can t count on it   so the user won t feel any delay  but if you call setState in reaction to an ajax call response or some other event that isn t triggered by the user  then the state might be updated with a slight delay  since the user won t really feel this delay  and it will improve performance by waiting to batch multiple state updates together and rerender the DOM fewer times

User · Answer

You can call a function after the state value has updated   this setState  foo   bar         gt            Do something here         Also  if you have lots of states to update at once  group them all within the same setState   Instead of   this setState  foo   one         gt        this setState  bar   two           Just do this   this setState       foo   one       bar   two

User · Answer

Imagine incrementing a counter in some component     class SomeComponent extends Component       state           updatedByDiv            updatedByBtn            counter  0            divCountHandler        gt          this setState           updatedByDiv   Div           counter  this state counter   1                 console log  divCountHandler executed               btnCountHandler        gt          this setState           updatedByBtn   Button           counter  this state counter   1                 console log  btnCountHandler executed                              render          return                          a parent div          lt div onClick  this divCountHandler  gt               a child button            lt button onClick  this btnCountHandler  gt Increment Count lt  button gt           lt  div gt                                  There is a count handler attached to both the parent and the child components  This is done purposely so we can execute the setState   twice within the same click event bubbling context  but from within 2 different handlers    As we would imagine  a single click event on the button would now trigger both these handlers since the event bubbles from target to the outermost container during the bubbling phase    Therefore the btnCountHandler   executes first  expected to increment the count to 1 and then the divCountHandler   executes  expected to increment the count to 2    However the count only increments to 1 as you can inspect in React Developer tools   This proves that react    queues all the setState calls comes back to this queue after executing the last method in the context the divCountHandler in this case  merges all the object mutations happening within multiple setState calls in the same context all method calls within a single event phase is same context for e g   into one single object mutation syntax  merging makes sense because this is why we can update the state properties independently in setState   in the first place  and passes it into one single setState   to prevent re-rendering due to multiple setState   calls  this is a very primitive description of batching      Resultant code run by react   this setState     updatedByDiv   Div     updatedByBtn   Button     counter  this state counter   1      To stop this behaviour  instead of passing objects as arguments to the setState method  callbacks are passed       divCountHandler        gt              this setState  prevState  props    gt                return                 updatedByDiv   Div                 counter  prevState counter   1                                        console log  divCountHandler executed                   btnCountHandler        gt              this setState  prevState  props    gt                return                 updatedByBtn   Button                 counter  prevState counter   1                                    console log  btnCountHandler executed            After the last method finishes execution and when react returns to process the setState queue  it simply calls the callback for each setState queued  passing in the previous component state   This way react ensures that the last callback in the queue gets to update the state that all of its previous counterparts have laid hands on

User · Answer

You can use the following wrap to make sync call  x000D   x000D  this setState  state   gt   x000D    return  x000D      something x000D      x000D     x000D   x000D   x000D

[javascript] Why is setState in reactjs Async instead of Sync?

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