The set function returned by useState lets you update the state to a different value and trigger a re-render. Set functions, like setSomething(nextState) The result from one of the calls will be ignored. If your initializer function is pure (as it should be), this should not affect the logic of your component. This is development-only behavior and does not affect production. In Strict Mode, React will call your initializer function twice in order to help you find accidental impurities.If you need that, extract a new component and move the state into it. You can’t call it inside loops or conditions. useState is a Hook, so you can only call it at the top level of your component or your own Hooks.The set function that lets you update the state to a different value and trigger a re-render.During the first render, it will match the initialState you have passed. UseState returns an array with exactly two values: React will call your initializer function when initializing the component, and store its return value as the initial state. It should be pure, should take no arguments, and should return a value of any type. If you pass a function as initialState, it will be treated as an initializer function.This argument is ignored after the initial render. It can be a value of any type, but there is a special behavior for functions. initialState: The value you want the state to be initially.The convention is to name state variables like using array destructuring. The rest of your component function will still execute (and the result will be thrown away), but if your condition is below all the calls to Hooks, you may add an early return inside it to restart rendering earlier.Ĭall useState at the top level of your component to declare a state variable. This way, children don’t need to render twice. When you call the set function during render, React will re-render that component immediately after your component exits with a return statement, and before rendering the children. However, it’s better than updating state in an effect. This pattern can be hard to understand and is usually best avoided. Finally, your set call should still update state without mutation - this special case doesn’t mean you can break other rules of pure functions. Calling the set function of another component during rendering is an error. Also, you can only update the state of the currently rendering component like this. Otherwise, your component would re-render in a loop until it crashes. Note that if you call a set function while rendering, it must be inside a condition like prevCount != count, and there must be a call like setPrevCount(count) inside of the condition. If it’s calculated from the previous state of some other state variable, you might want to combine them into one object and use a reducer. If you prefer consistency over slightly more verbose syntax, it’s reasonable to always write an updater if the state you’re setting is calculated from the previous state. They’re also helpful if accessing the state variable itself is inconvenient (you might run into this when optimizing re-renders). However, if you do multiple updates within the same event, updaters can be helpful. This means there is no risk of a click handler seeing a “stale” age at the beginning of the event handler. React always makes sure that for intentional user actions, like clicks, the age state variable would be updated before the next click. In most cases, there is no difference between these two approaches. There is no harm in it, but it is also not always necessary. #Unpkg import setstate from react codeHowever, the spread syntax simplifies this operation and it also works with arrays.You might hear a recommendation to always write code like setAge(a => a + 1) if the state you’re setting is calculated from the previous state. There are two types of components in React: class and functional components.Ĭlass components are ES6 classes that extend from React.Component and can have state and lifecycle methods: class Message extends React.Component ) If you’re just getting started with React Hooks and looking for a visual guide, check out the video tutorial below.
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