How to use componentWillMount in React Hooks

Question

In the official docs of React it mentions -       If you   re familiar with React class lifecycle methods  you can think   of useEffect Hook as componentDidMount  componentDidUpdate  and   componentWillUnmount combined    My question is - how can we use the componentWillMount   lifecyle method in a hook

User · Answer

https   reactjs org docs hooks-reference html usememo     Remember that the function passed to useMemo runs during rendering    Don   t do anything there that you wouldn   t normally do while rendering    For example  side effects belong in useEffect  not useMemo

User · Answer

React Lifecycle methods in hooks for simple visual reference follow this image  As you can simply see in the above picture that for ComponentWillUnmount  you have to do like this  useEffect      gt        return      gt            console log  componentWillUnmount

User · Answer

You can hack the useMemo hook to imitate a componentWillMount lifecycle event  Just do  const Component        gt       useMemo      gt            componentWillMount events              useEffect      gt            componentDidMount events      return      gt              componentWillUnmount events                       You would need to keep the useMemo hook before anything that interacts with your state  This is not how it is intended but it worked for me for all componentWillMount issues  This works because useMemo doesnt require to actually return a value and you dont have to actually use it as anything  but since it memorizes a value based on dependencies which will only run once   quot    quot   and its on top of our component it runs once when the component mounts before anything else

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Just simply add an empty dependenncy array in useEffect it will works as componentDidMount   useEffect      gt         Your code here   console log  componentDidMount

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You cannot use any of the existing lifecycle methods  componentDidMount  componentDidUpdate  componentWillUnmount etc   in a hook  They can only be used in class components  And with Hooks you can only use in functional components  The line below comes from the React doc      If you   re familiar with React class lifecycle methods  you can think of useEffect Hook as componentDidMount  componentDidUpdate  and componentWillUnmount combined    suggest is  you can mimic these lifecycle method from class component in a functional components   Code inside componentDidMount run once when the component is mounted  useEffect hook equivalent for this behaviour is   useEffect      gt         Your code here           Notice the second parameter here  empty array   This will run only once   Without the second parameter the useEffect hook will be called on every render of the component which can be dangerous   useEffect      gt         Your code here       componentWillUnmount is use for cleanup  like removing event listeners  cancel the timer etc   Say you are adding a event listener in componentDidMount and removing it in componentWillUnmount as below   componentDidMount       window addEventListener  mousemove        gt         componentWillUnmount       window removeEventListener  mousemove        gt          Hook equivalent of above code will be as follows  useEffect      gt      window addEventListener  mousemove        gt             returned function will be called on component unmount    return      gt        window removeEventListener  mousemove        gt

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This is the way how I simulate constructor in functional components using the useRef hook   function Component props        const willMount   useRef true       if  willMount current            console log  This runs only once before rendering the component             willMount current   false                     return   lt h1 gt Meow world  lt  h1 gt        Here is the lifecycle example   function RenderLog props        console log  Render log      props children       return   lt  gt  props children  lt   gt       function Component props         console log  Body        const  count  setCount    useState 0       const willMount   useRef true        if  willMount current            console log  First time load  it runs only once             setCount 2           willMount current   false        else           console log  Repeated load               useEffect      gt            console log  Component did mount  it runs only once             return      gt  console log  Component will unmount                     useEffect      gt            console log  Component did update                 useEffect      gt            console log  Component will receive props            count          return            lt  gt           lt h1 gt  count  lt  h1 gt           lt RenderLog gt  count  lt  RenderLog gt           lt   gt              Log  Body  Log  First time load  it runs only once   Log  Body  Log  Repeated load  Log  Render log  2  Log  Component did mount  it runs only once   Log  Component did update  Log  Component will receive props   Of course Class components don t have Body steps  it s not possible to make 1 1 simulation due to different concepts of functions and classes

User · Answer

There is a nice workaround to implement componentDidMount and componentWillUnmount with useEffect   Based on the documentation  useEffect can return a  cleanup  function  this function will not be invoked on the first useEffect call  only on subsequent calls   Therefore  if we use the useEffect hook with no dependencies at all  the hook will be called only when the component is mounted and the  cleanup  function is called when the component is unmounted   useEffect      gt        console log  componentDidMount         return      gt            console log  componentWillUnmount                     The cleanup return function call is invoked only when the component is unmounted   Hope this helps

User · Answer

useComponentWillMount hook export const useComponentWillMount    func    gt        const willMount   useRef true       if  willMount current  func        willMount current   false       or export const useComponentWillMount    func    gt        useMemo func         Discussion In class components componentWillMount is considered legacy  source 1  source2   However  this shouldn t apply to functional components and a hook based solution  Class component componentWillMount is deprecated since it might run more than once  and there is an alternative - using the constructor  Those considerations aren t relevant for a functional component  My experience is that such a hook could be a saver when timing sequence is critical  I m interested to know what is your use case - comments are welcome  useComponentDidMount hook Alternatively  use componentDidMount hook  const useComponentDidMount   func   gt  useEffect func        It will run only once  after component has mounted initial render to the dom   Demo const Component    props    gt      useComponentWillMount      gt  console log  quot Runs only once before component mounts quot       useComponentDidMount      gt  console log  quot Runs only once after component mounts quot              return        lt div gt       lt  div gt          Full Demo

User · Answer

useLayoutEffect could accomplish this with an empty set of observers      if the functionality is actually similar to componentWillMount -- it will run before the first content gets to the DOM -- though there are actually two updates but they are synchronous before drawing to the screen   for example    function MyComponent      andItsProps           useLayoutEffect     gt              console log  I am about to render                       return   lt div gt some content lt  div gt         The benefit over useState with an initializer setter or useEffect is though it may compute a render pass  there are no actual re-renders to the DOM that a user will notice  and it is run before the first noticable render  which is not the case for useEffect  The downside is of course a slight delay in your first render since a check update has to happen before painting to screen  It really does depend on your use-case  though    I think personally  useMemo is fine in some niche cases where you need to do something heavy -- as long as you keep in mind it is the exception vs the norm

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It might be clear for most  but have in mind that a function called inside the function component s body  acts as a beforeRender  This doesn t answer the question of running code on ComponentWillMount  before the first render  but since it is related and might help others I m leaving it here  const MyComponent        gt      const  counter  setCounter    useState 0       useEffect      gt        console log  after render           const iterate        gt        setCounter prevCounter   gt  prevCounter 1         const beforeRender        gt        console log  before render          beforeRender      return        lt div gt         lt div gt  counter  lt  div gt         lt button onClick  iterate  gt Re-render lt  button gt       lt  div gt         export default MyComponent

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I wrote a custom hook that will run a function once before first render   useBeforeFirstRender js  import   useState  useEffect   from  react   export default  fun    gt      const  hasRendered  setHasRendered    useState false     useEffect      gt  setHasRendered true    hasRendered      if   hasRendered        fun           Usage   import React    useEffect   from  react  import useBeforeFirstRender from     hooks useBeforeFirstRender    export default      gt       useBeforeFirstRender      gt        console log  Do stuff here           return        lt div gt        My component      lt  div gt

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Ben Carp s answer seems like only valid one to me   But since we are using functional ways just another approach can be benefiting from closure and HoC    const InjectWillmount   function Node  willMountCallback      let isCalled   true    return function         if  isCalled          willMountCallback          isCalled   false            return Node            Then use it    const YourNewComponent   InjectWillmount  lt YourComponent   gt        gt      console log  your pre-mount logic here

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According to reactjs org  componentWillMount will not be supported in the future  https   reactjs org docs react-component html unsafe componentwillmount There is no need to use componentWillMount  If you want to do something before the component mounted  just do it in the constructor    If you want to do network requests  do not do it in componentWillMount  It is because doing this will lead to unexpected bugs  Network requests can be done in componentDidMount  Hope it helps   updated on 08 03 2019 The reason why you ask for componentWillMount is probably because you want to initialize the state before renders  Just do it in useState  const helloWorld     gt       const  value setValue  useState 0    initialize your state here     return  lt p gt  value  lt  p gt    export default helloWorld   or maybe You want to run a function in componentWillMount  for example  if your original code looks like this  componentWillMount      console log  componentWillMount      with hook  all you need to do is to remove the lifecycle method  const hookComponent     gt       console log  componentWillMount       return  lt p gt you have transfered componeWillMount from class component into hook  lt  p gt     I just want to add something to the first answer about useEffect  useEffect     gt      useEffect runs on every render  it is a combination of componentDidUpdate  componentDidMount and ComponentWillUnmount   useEffect     gt         If we add an empty array in useEffect it runs just when the component mounted  It is because useEffect will compare the array you passed to it  So it does not have to be an empty array It can be array that is not changing  For example  it can be  1 2 3  or   1 2    useEffect still only runs when component mounted  It depends on you whether you want it to run just once or runs after every render  It is not dangerous if you forgot to add an array as long as you know what you are doing  I created a sample for hook  Please check it out  https   codesandbox io s kw6xj153wr  updated on 21 08 2019 It has been a while since I wrote the above answer  There is something that I think you need to pay attention to  When you use useEffect     gt         When react compares the values you passed to the array     it uses Object is   to compare  If you pass an object to it  such as useEffect     gt      name  Tom      This is exactly the same as  useEffect     gt      It will re-render every time because when Object is   compares an object  it compares its reference  not the value itself  It is the same as why         returns false because their references are different  If you still want to compare the object itself not the reference  you can do something like this  useEffect     gt     JSON stringify  name  Tom

User · Answer

Short answer to your original question  how componentWillMount can be used with React Hooks   componentWillMount is deprecated and considered legacy  React recommendation      Generally  we recommend using the constructor   instead for initializing state    Now in the Hook FAQ you find  out  what the equivalent of a class constructor  for function components is      constructor  Function components don   t need a constructor  You can initialize the state in the useState call  If computing the initial state is expensive  you can pass a function to useState    So a usage example of componentWillMount looks like this   const MyComp        gt      const  state  setState    useState 42     set initial value directly in useState    const  state2  setState2    useState createInitVal     call complex computation    return  lt div gt  state   state2  lt  div gt      const createInitVal        gt           complex computation or other logic    return 42

User · Answer

As it has been stated in react document   You might be thinking that we   d need a separate effect to perform the cleanup  But code for adding and removing a subscription is so tightly related that useEffect is designed to keep it together  If your effect returns a function  React will run it when it is time to clean up   useEffect      gt        function handleStatusChange status          setIsOnline status isOnline             ChatAPI subscribeToFriendStatus props friend id  handleStatusChange          Specify how to clean up after this effect      return function cleanup           ChatAPI unsubscribeFromFriendStatus props friend id  handleStatusChange                   if  isOnline     null        return  Loading            return isOnline    Online     Offline      so the only thing that we need to have the componentWillUnmount in hooks is to return a function inside a useEffect  as explained above

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