React Tutorial

React Tutorial

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React Tutorial for Beginners - Hooks API Reference

Hooks API Reference

Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.

This page describes the APIs for the built-in Hooks in React.

If you’re new to Hooks, you might want to check out the overview first. You may also find useful information in the frequently asked questions section.

  • Basic Hooks
    • useState
    • useEffect
    • useContext
  • Additional Hooks
    • useReducer
    • useCallback
    • useMemo
    • useRef
    • useImperativeHandle
    • useLayoutEffect
    • useDebugValue

Basic Hooks

useState

const [state, setState] = useState(initialState);

Returns a stateful value, and a function to update it.

During the initial render, the returned state (state) is the same as the value passed as the first argument (initialState).

The setState function is used to update the state. It accepts a new state value and enqueues a re-render of the component.

setState(newState);

During subsequent re-renders, the first value returned by useState will always be the most recent state after applying updates.

Note

React guarantees that setState function identity is stable and won’t change on re-renders. This is why it’s safe to omit from the useEffect or useCallback dependency list.

Functional updates

If the new state is computed using the previous state, you can pass a function to setState. The function will receive the previous value, and return an updated value. Here’s an example of a counter component that uses both forms of setState:

function Counter({initialCount}) {
  const [count, setCount] = useState(initialCount);
  return (
    <>
      Count: {count}
      <button onClick={() => setCount(initialCount)}>Reset</button>
      <button onClick={() => setCount(prevCount => prevCount - 1)}>-</button>
      <button onClick={() => setCount(prevCount => prevCount + 1)}>+</button>
    </>
  );
}

The ”+” and ”-” buttons use the functional form, because the updated value is based on the previous value. But the “Reset” button uses the normal form, because it always sets the count back to the initial value.

If your update function returns the exact same value as the current state, the subsequent rerender will be skipped completely.

Note

Unlike the setState method found in class components, useState does not automatically merge update objects. You can replicate this behavior by combining the function updater form with object spread syntax:

setState(prevState => {
  // Object.assign would also work
  return {...prevState, ...updatedValues};
});

Another option is useReducer, which is more suited for managing state objects that contain multiple sub-values.

Lazy initial state

The initialState argument is the state used during the initial render. In subsequent renders, it is disregarded. If the initial state is the result of an expensive computation, you may provide a function instead, which will be executed only on the initial render:

const [state, setState] = useState(() => {
  const initialState = someExpensiveComputation(props);
  return initialState;
});
Bailing out of a state update

If you update a State Hook to the same value as the current state, React will bail out without rendering the children or firing effects. (React uses the Object.is comparison algorithm.)

Note that React may still need to render that specific component again before bailing out. That shouldn’t be a concern because React won’t unnecessarily go “deeper” into the tree. If you’re doing expensive calculations while rendering, you can optimize them with useMemo.

useEffect

useEffect(didUpdate);

Accepts a function that contains imperative, possibly effectful code.

Mutations, subscriptions, timers, logging, and other side effects are not allowed inside the main body of a function component (referred to as React’s render phase). Doing so will lead to confusing bugs and inconsistencies in the UI.

Instead, use useEffect. The function passed to useEffect will run after the render is committed to the screen. Think of effects as an escape hatch from React’s purely functional world into the imperative world.

By default, effects run after every completed render, but you can choose to fire them only when certain values have changed.

Cleaning up an effect

Often, effects create resources that need to be cleaned up before the component leaves the screen, such as a subscription or timer ID. To do this, the function passed to useEffect may return a clean-up function. For example, to create a subscription:

useEffect(() => {
  const subscription = props.source.subscribe();
  return () => {
    // Clean up the subscription
    subscription.unsubscribe();
  };
});

The clean-up function runs before the component is removed from the UI to prevent memory leaks. Additionally, if a component renders multiple times (as they typically do), the previous effect is cleaned up before executing the next effect. In our example, this means a new subscription is created on every update. To avoid firing an effect on every update, refer to the next section.

Timing of effects

Unlike componentDidMount and componentDidUpdate, the function passed to useEffect fires after layout and paint, during a deferred event. This makes it suitable for the many common side effects, like setting up subscriptions and event handlers, because most types of work shouldn’t block the browser from updating the screen.

However, not all effects can be deferred. For example, a DOM mutation that is visible to the user must fire synchronously before the next paint so that the user does not perceive a visual inconsistency. (The distinction is conceptually similar to passive versus active event listeners.) For these types of effects, React provides one additional Hook called useLayoutEffect. It has the same signature as useEffect, and only differs in when it is fired.

Although useEffect is deferred until after the browser has painted, it’s guaranteed to fire before any new renders. React will always flush a previous render’s effects before starting a new update.

Conditionally firing an effect

The default behavior for effects is to fire the effect after every completed render. That way an effect is always recreated if one of its dependencies changes.

However, this may be overkill in some cases, like the subscription example from the previous section. We don’t need to create a new subscription on every update, only if the source prop has changed.

To implement this, pass a second argument to useEffect that is the array of values that the effect depends on. Our updated example now looks like this:

useEffect(
  () => {
    const subscription = props.source.subscribe();
    return () => {
      subscription.unsubscribe();
    };
  },
  [props.source],
);

Now the subscription will only be recreated when props.source changes.

Note

If you use this optimization, make sure the array includes all values from the component scope (such as props and state) that change over time and that are used by the effect. Otherwise, your code will reference stale values from previous renders. Learn more about how to deal with functions and what to do when the array values change too often.

If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array ([]) as a second argument. This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run. This isn’t handled as a special case — it follows directly from how the dependencies array always works.

If you pass an empty array ([]), the props and state inside the effect will always have their initial values. While passing [] as the second argument is closer to the familiar componentDidMount and componentWillUnmount mental model, there are usually better solutions to avoid re-running effects too often. Also, don’t forget that React defers running useEffect until after the browser has painted, so doing extra work is less of a problem.

We recommend using the exhaustive-depsrule as part of oureslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.

The array of dependencies is not passed as arguments to the effect function. Conceptually, though, that’s what they represent: every value referenced inside the effect function should also appear in the dependencies array. In the future, a sufficiently advanced compiler could create this array automatically.

useContext

const value = useContext(MyContext);

Accepts a context object (the value returned from React.createContext) and returns the current context value for that context. The current context value is determined by the value prop of the nearest <MyContext.Provider> above the calling component in the tree.

When the nearest <MyContext.Provider> above the component updates, this Hook will trigger a rerender with the latest context value passed to that MyContext provider. Even if an ancestor uses React.memoorshouldComponentUpdate, a rerender will still happen starting at the component itself using useContext.

Don’t forget that the argument to useContext must be the context object itself:

  • Correct: useContext(MyContext)
  • Incorrect: useContext(MyContext.Consumer)
  • Incorrect: useContext(MyContext.Provider)

A component calling useContext will always re-render when the context value changes. If re-rendering the component is expensive, you can optimize it by using memoization.

Tip

If you’re familiar with the context API before Hooks, useContext(MyContext) is equivalent to static contextType = MyContext in a class, or to <MyContext.Consumer>.

useContext(MyContext) only lets you read the context and subscribe to its changes. You still need a <MyContext.Provider> above in the tree to provide the value for this context.

Putting it together with Context.Provider

const themes = {
  light: {
    foreground: "#000000",
    background: "#eeeeee"
  },
  dark: {
    foreground: "#ffffff",
    background: "#222222"
  }
};

const ThemeContext = React.createContext(themes.light);

function App() {
  return (
    <ThemeContext.Provider value={themes.dark}>
      <Toolbar />
    </ThemeContext.Provider>
  );
}

function Toolbar(props) {
  return (
    <div>
      <ThemedButton />
    </div>
  );
}

function ThemedButton() {
  const theme = useContext(ThemeContext);  return (    <button style={{ background: theme.background, color: theme.foreground }}>      I am styled by theme context!    </button>  );
}

This example is modified for hooks from a previous example in the Context Advanced Guide, where you can find more information about when and how to use Context.

Additional Hooks

The following Hooks are either variants of the basic ones from the previous section, or only needed for specific edge cases. Don’t stress about learning them up front.

useReducer

const [state, dispatch] = useReducer(reducer, initialArg, init);

An alternative to useState. Accepts a reducer of type (state, action) => newState, and returns the current state paired with a dispatch method. (If you’re familiar with Redux, you already know how this works.)

useReducer is usually preferable to useState when you have complex state logic that involves multiple sub-values or when the next state depends on the previous one. useReducer also lets you optimize performance for components that trigger deep updates because you can pass dispatch down instead of callbacks.

Here’s the counter example from the useState section, rewritten to use a reducer:

const initialState = {count: 0};

function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return {count: state.count + 1};
    case 'decrement':
      return {count: state.count - 1};
    default:
      throw new Error();
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);
  return (
    <>
      Count: {state.count}
      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
      <button onClick={() => dispatch({type: 'increment'})}>+</button>
    </>
  );
}

Note

React guarantees that dispatch function identity is stable and won’t change on re-renders. This is why it’s safe to omit from the useEffect or useCallback dependency list.

Specifying the initial state

There are two different ways to initialize useReducer state. You may choose either one depending on the use case. The simplest way is to pass the initial state as a second argument:

  const [state, dispatch] = useReducer(
    reducer,
    {count: initialCount}  );

Note

React doesn’t use the state = initialState argument convention popularized by Redux. The initial value sometimes needs to depend on props and so is specified from the Hook call instead. If you feel strongly about this, you can call useReducer(reducer, undefined, reducer) to emulate the Redux behavior, but it’s not encouraged.

Lazy initialization

You can also create the initial state lazily. To do this, you can pass an init function as the third argument. The initial state will be set to init(initialArg).

It lets you extract the logic for calculating the initial state outside the reducer. This is also handy for resetting the state later in response to an action:

function init(initialCount) {  return {count: initialCount};}
function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return {count: state.count + 1};
    case 'decrement':
      return {count: state.count - 1};
    case 'reset':      return init(action.payload);    default:
      throw new Error();
  }
}

function Counter({initialCount}) {
  const [state, dispatch] = useReducer(reducer, initialCount, init);  return (
    <>
      Count: {state.count}
      <button
        onClick={() => dispatch({type: 'reset', payload: initialCount})}>        Reset
      </button>
      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
      <button onClick={() => dispatch({type: 'increment'})}>+</button>
    </>
  );
}
Bailing out of a dispatch

If you return the same value from a Reducer Hook as the current state, React will bail out without rendering the children or firing effects. (React uses the Object.is comparison algorithm.)

Note that React may still need to render that specific component again before bailing out. That shouldn’t be a concern because React won’t unnecessarily go “deeper” into the tree. If you’re doing expensive calculations while rendering, you can optimize them with useMemo.

useCallback

const memoizedCallback = useCallback(
  () => {
    doSomething(a, b);
  },
  [a, b],
);

Returns a memoized callback.

Pass an inline callback and an array of dependencies. useCallback will return a memoized version of the callback that only changes if one of the dependencies has changed. This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders (e.g. shouldComponentUpdate).

useCallback(fn, deps) is equivalent to useMemo(() => fn, deps).

Note

The array of dependencies is not passed as arguments to the callback. Conceptually, though, that’s what they represent: every value referenced inside the callback should also appear in the dependencies array. In the future, a sufficiently advanced compiler could create this array automatically.

We recommend using the exhaustive-depsrule as part of oureslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.

useMemo

const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

Returns a memoized value.

Pass a “create” function and an array of dependencies. useMemo will only recompute the memoized value when one of the dependencies has changed. This optimization helps to avoid expensive calculations on every render.

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.

If no array is provided, a new value will be computed on every render.

You may rely on **useMemo** as a performance optimization, not as a semantic guarantee. In the future, React may choose to “forget” some previously memoized values and recalculate them on next render, e.g. to free memory for offscreen components. Write your code so that it still works without useMemo — and then add it to optimize performance.

Note

The array of dependencies is not passed as arguments to the function. Conceptually, though, that’s what they represent: every value referenced inside the function should also appear in the dependencies array. In the future, a sufficiently advanced compiler could create this array automatically.

We recommend using the exhaustive-depsrule as part of oureslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.

useRef

const refContainer = useRef(initialValue);

useRef returns a mutable ref object whose .current property is initialized to the passed argument (initialValue). The returned object will persist for the full lifetime of the component.

A common use case is to access a child imperatively:

function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

Essentially, useRef is like a “box” that can hold a mutable value in its .current property.

You might be familiar with refs primarily as a way to access the DOM. If you pass a ref object to React with <div ref={myRef} />, React will set its .current property to the corresponding DOM node whenever that node changes.

However, useRef() is useful for more than the ref attribute. It’s handy for keeping any mutable value around similar to how you’d use instance fields in classes.

This works because useRef() creates a plain JavaScript object. The only difference between useRef() and creating a {current: ...} object yourself is that useRef will give you the same ref object on every render.

Keep in mind that useRef doesn’t notify you when its content changes. Mutating the .current property doesn’t cause a re-render. If you want to run some code when React attaches or detaches a ref to a DOM node, you may want to use a callback ref instead.

useImperativeHandle

useImperativeHandle(ref, createHandle, [deps])

useImperativeHandle customizes the instance value that is exposed to parent components when using ref. As always, imperative code using refs should be avoided in most cases. useImperativeHandle should be used with forwardRef:

function FancyInput(props, ref) {
  const inputRef = useRef();
  useImperativeHandle(ref, () => ({
    focus: () => {
      inputRef.current.focus();
    }
  }));
  return <input ref={inputRef} ... />;
}
FancyInput = forwardRef(FancyInput);

In this example, a parent component that renders <FancyInput ref={inputRef} /> would be able to call inputRef.current.focus().

useLayoutEffect

The signature is identical to useEffect, but it fires synchronously after all DOM mutations. Use this to read layout from the DOM and synchronously re-render. Updates scheduled inside useLayoutEffect will be flushed synchronously, before the browser has a chance to paint.

Prefer the standard useEffect when possible to avoid blocking visual updates.

Tip

If you’re migrating code from a class component, note useLayoutEffect fires in the same phase as componentDidMount and componentDidUpdate. However, we recommend starting with **useEffect** first and only trying useLayoutEffect if that causes a problem.

If you use server rendering, keep in mind that neither useLayoutEffect nor useEffect can run until the JavaScript is downloaded. This is why React warns when a server-rendered component contains useLayoutEffect. To fix this, either move that logic to useEffect (if it isn’t necessary for the first render), or delay showing that component until after the client renders (if the HTML looks broken until useLayoutEffect runs).

To exclude a component that needs layout effects from the server-rendered HTML, render it conditionally with showChild && <Child /> and defer showing it with useEffect(() => { setShowChild(true); }, []). This way, the UI doesn’t appear broken before hydration.

useDebugValue

useDebugValue(value)

useDebugValue can be used to display a label for custom hooks in React DevTools.

For example, consider the useFriendStatus custom Hook described in “Building Your Own Hooks”:

function useFriendStatus(friendID) {
  const [isOnline, setIsOnline] = useState(null);

  // ...

  // Show a label in DevTools next to this Hook  // e.g. "FriendStatus: Online"  useDebugValue(isOnline ? 'Online' : 'Offline');
  return isOnline;
}

Tip

We don’t recommend adding debug values to every custom Hook. It’s most valuable for custom Hooks that are part of shared libraries.

Defer formatting debug values

In some cases formatting a value for display might be an expensive operation. It’s also unnecessary unless a Hook is actually inspected.

For this reason useDebugValue accepts a formatting function as an optional second parameter. This function is only called if the Hooks are inspected. It receives the debug value as a parameter and should return a formatted display value.

For example a custom Hook that returned a Date value could avoid calling the toDateString function unnecessarily by passing the following formatter:

useDebugValue(date, date => date.toDateString());

#react #api #javascript #web-development #developer

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React Tutorial for Beginners - Hooks API Reference
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If you are undertaking a mobile app development for your start-up or enterprise, you are likely wondering whether to use React Native. As a popular development framework, React Native helps you to develop near-native mobile apps. However, you are probably also wondering how close you can get to a native app by using React Native. How native is React Native?

In the article, we discuss the similarities between native mobile development and development using React Native. We also touch upon where they differ and how to bridge the gaps. Read on.

A brief introduction to React Native

Let’s briefly set the context first. We will briefly touch upon what React Native is and how it differs from earlier hybrid frameworks.

React Native is a popular JavaScript framework that Facebook has created. You can use this open-source framework to code natively rendering Android and iOS mobile apps. You can use it to develop web apps too.

Facebook has developed React Native based on React, its JavaScript library. The first release of React Native came in March 2015. At the time of writing this article, the latest stable release of React Native is 0.62.0, and it was released in March 2020.

Although relatively new, React Native has acquired a high degree of popularity. The “Stack Overflow Developer Survey 2019” report identifies it as the 8th most loved framework. Facebook, Walmart, and Bloomberg are some of the top companies that use React Native.

The popularity of React Native comes from its advantages. Some of its advantages are as follows:

  • Performance: It delivers optimal performance.
  • Cross-platform development: You can develop both Android and iOS apps with it. The reuse of code expedites development and reduces costs.
  • UI design: React Native enables you to design simple and responsive UI for your mobile app.
  • 3rd party plugins: This framework supports 3rd party plugins.
  • Developer community: A vibrant community of developers support React Native.

Why React Native is fundamentally different from earlier hybrid frameworks

Are you wondering whether React Native is just another of those hybrid frameworks like Ionic or Cordova? It’s not! React Native is fundamentally different from these earlier hybrid frameworks.

React Native is very close to native. Consider the following aspects as described on the React Native website:

  • Access to many native platforms features: The primitives of React Native render to native platform UI. This means that your React Native app will use many native platform APIs as native apps would do.
  • Near-native user experience: React Native provides several native components, and these are platform agnostic.
  • The ease of accessing native APIs: React Native uses a declarative UI paradigm. This enables React Native to interact easily with native platform APIs since React Native wraps existing native code.

Due to these factors, React Native offers many more advantages compared to those earlier hybrid frameworks. We now review them.

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In this article, you will learn what are hooks in React JS? and when to use react hooks? React JS is developed by Facebook in the year 2013. There are many students and the new developers who have confusion between react and hooks in react. Well, it is not different, react is a programming language and hooks is a function which is used in react programming language.
Read More:- https://infoatone.com/what-are-hooks-in-react-js/

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What is an API? Learn all about APIs (Application Programming Interfaces) in this full tutorial for beginners. You will learn what APIs do, why APIs exist, and the many benefits of APIs. APIs are used all the time in programming and web development so it is important to understand how to use them.

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When working with location services, it is important that the information you collect is accurate for your users or clients. This will prevent any mistakes in shipping, billing, and many other aspects of operations that rely on correct location information. For businesses that have applications using location services, this is especially important as any incorrect data can mean the displacement of goods or interrupted services.

The following APIs will allow you to fully validate street addresses by first parsing address data input and then verifying and normalizing the information. The last two APIs will also allow you to geocode and reverse geocode an address to receive more accurate location data for your applications.

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The following APIs will allow you to instantly verify and validate an input email address without sending any kind of notification to the email user. This will help protect your organization in the event of any threats. The goal of this tutorial is to provide you with the tools to protect your organization’s information while providing a way to verify new accounts and user information.

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