When to use React setState callback

Question

When a react component state changes  the render method is called  Hence for any state change  an action can be performed in the render methods body  Is there a particular use case for the setState callback then

User · Answer

The 1  usecase which comes into my mind  is an api call  which should t go into the render  because it will run for each state change  And the API call should be only performed on special state change  and not on every render   changeSearchParams    params    gt      this setState   params    this performSearch      performSearch        gt      API search this state params   result    gt        this setState   result                 Hence for any state change  an action can be performed in the render methods body    Very bad practice  because the render-method should be pure  it means no actions  state changes  api calls  should be performed  just composite your view and return it  Actions should be performed on some events only  Render is not an event  but componentDidMount for example

User · Answer

Sometimes we need a code block where we need to perform some operation right after setState where we are sure the state is being updated  That is where setState callback comes into play For example  there was a scenario where I needed to enable a modal for 2 customers out of 20 customers  for the customers where we enabled it  there was a set of time taking API calls  so it looked like this async componentDidMount       const appConfig   getCustomerConfig      this setState  enableModal  appConfig  enableFeatures  paymentModal    async         gt        if this state enableModal            make some API call for data needed in poput                 enableModal boolean was required in UI blocks in the render function as well   that s why I did setState here  otherwise  could ve just checked condition once and either called API set or not

User · Answer

Yes there is  since setState works in an asynchronous way  That means after calling setState the this state variable is not immediately changed  so if you want to perform an action immediately after setting state on a state variable and then return a result  a callback will be useful  Consider the example below       changeTitle  function changeTitle  event      this setState   title  event target value       this validateTitle       validateTitle  function validateTitle        if  this state title length     0        this setState   titleError   Title can t be blank                    The above code may not work as expected since the title variable may not have mutated before validation is performed on it  Now you may wonder that we can perform the validation in the render   function itself but it would be better and a cleaner way if we can handle this in the changeTitle function itself since that would make your code more organised and understandable  In this case callback is useful        changeTitle  function changeTitle  event      this setState   title  event target value    function         this validateTitle              validateTitle  function validateTitle        if  this state title length     0        this setState   titleError   Title can t be blank                    Another example will be when you want to dispatch and action when the state changed  you will want to do it in a callback and not the render   as it will be called everytime rerendering occurs and hence many such scenarios are possible where you will need callback   Another case is a API Call  A case may arise when you need to make an API call based on a particular state change  if you do that in the render method  it will be called on every render onState change or because some Prop passed down to the Child Component changed   In this case you would want to use a setState callback to pass the updated state value to the API call       changeTitle  function  event      this setState   title  event target value         gt  this APICallFunction        APICallFunction  function           Call API with the updated value

User · Answer

Consider setState call  this setState   counter  this state counter   1      IDEA     setState may be called in async function   So you cannot rely on this  If the above call was made inside a async function this will refer to state of component at that point of time but we expected this to refer to property inside state at time setState calling or beginning of async task  And as task was async call thus that property may have changed in time being  Thus it is unreliable to use this keyword to refer to some property of state thus we use callback function whose arguments are previousState and props which means when async task was done and it was time to update state using setState call prevState will refer to state now when setState has not started yet  Ensuring reliability that nextState would not be corrupted   Wrong Code  would lead to corruption of data  this setState      counter this state counter 1        Correct Code with setState  having call back function    this setState          prevState props   gt              return  counter prevState counter 1                      Thus whenever we need to update our current state to next state based on value possed by property just now and all this is happening in async fashion it is good idea to use setState as callback function   I have tried to explain it in codepen here CODE PEN

User · Answer

this setState       name  value          gt        console log this state name

[reactjs] When to use React setState callback

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