API viite
Komponentit

Common components (e.g. <div>)

Kaikki selaimeen sisäänrakennetut komponentit, kuten <div>, tukevat joitakin yleisiä propseja ja tapahtumia.

Viite

Yleiset komponentit (kuten <div>) 

<div className="wrapper">Jotain sisältöä</div>

Näe lisää esimerkkejä alla.

Propsit

These special React props are supported for all built-in components:

  • children: A React node (an element, a string, a number, a portal, an empty node like null, undefined and booleans, or an array of other React nodes). Specifies the content inside the component. When you use JSX, you will usually specify the children prop implicitly by nesting tags like <div><span /></div>.

  • dangerouslySetInnerHTML: An object of the form { __html: '<p>some html</p>' } with a raw HTML string inside. Overrides the innerHTML property of the DOM node and displays the passed HTML inside. This should be used with extreme caution! If the HTML inside isn’t trusted (for example, if it’s based on user data), you risk introducing an XSS vulnerability. Read more about using dangerouslySetInnerHTML.

  • ref: A ref object from useRef or createRef, or a ref callback function, or a string for legacy refs. Your ref will be filled with the DOM element for this node. Read more about manipulating the DOM with refs.

  • suppressContentEditableWarning: Totuusarvo. If true, suppresses the warning that React shows for elements that both have children and contentEditable={true} (which normally do not work together). Use this if you’re building a text input library that manages the contentEditable content manually.

  • suppressHydrationWarning: Totuusarvo. If you use server rendering, normally there is a warning when the server and the client render different content. In some rare cases (like timestamps), it is very hard or impossible to guarantee an exact match. If you set suppressHydrationWarning to true, React will not warn you about mismatches in the attributes and the content of that element. It only works one level deep, and is intended to be used as an escape hatch. Don’t overuse it. Read about suppressing hydration errors.

  • style: An object with CSS styles, for example { fontWeight: 'bold', margin: 20 }. Similarly to the DOM style property, the CSS property names need to be written as camelCase, for example fontWeight instead of font-weight. You can pass strings or numbers as values. If you pass a number, like width: 100, React will automatically append px (“pixels”) to the value unless it’s a unitless property. We recommend using style only for dynamic styles where you don’t know the style values ahead of time. In other cases, applying plain CSS classes with className is more efficient. Read more about className and style.

These standard DOM props are also supported for all built-in components:

You can also pass custom attributes as props, for example mycustomprop="someValue". This can be useful when integrating with third-party libraries. The custom attribute name must be lowercase and must not start with on. The value will be converted to a string. If you pass null or undefined, the custom attribute will be removed.

These events fire only for the <form> elements:

These events fire only for the <dialog> elements. Unlike browser events, they bubble in React:

These events fire only for the <details> elements. Unlike browser events, they bubble in React:

These events fire for <img>, <iframe>, <object>, <embed>, <link>, and SVG <image> elements. Unlike browser events, they bubble in React:

These events fire for resources like <audio> and <video>. Unlike browser events, they bubble in React:

Rajoitukset

  • You cannot pass both children and dangerouslySetInnerHTML at the same time.
  • Some events (like onAbort and onLoad) don’t bubble in the browser, but bubble in React.

ref callback-funktio

Instead of a ref object (like the one returned by useRef), you may pass a function to the ref attribute.

<div ref={(node) => console.log(node)} />

See an example of using the ref callback.

When the <div> DOM node is added to the screen, React will call your ref callback with the DOM node as the argument. When that <div> DOM node is removed, React will call your ref callback with null.

React will also call your ref callback whenever you pass a different ref callback. In the above example, (node) => { ... } is a different function on every render. When your component re-renders, the previous function will be called with null as the argument, and the next function will be called with the DOM node.

Parametrit

  • node: A DOM node or null. React will pass you the DOM node when the ref gets attached, and null when the ref gets detached. Unless you pass the same function reference for the ref callback on every render, the callback will get temporarily detached and re-attached during every re-render of the component.

Palautukset

Do not return anything from the ref callback.

React tapahtumaolio

Your event handlers will receive a React event object. It is also sometimes known as a “synthetic event”.

<button onClick={e => {
  console.log(e); // React event object
}} />

It conforms to the same standard as the underlying DOM events, but fixes some browser inconsistencies.

Some React events do not map directly to the browser’s native events. For example in onMouseLeave, e.nativeEvent will point to a mouseout event. The specific mapping is not part of the public API and may change in the future. If you need the underlying browser event for some reason, read it from e.nativeEvent.

Ominaisuudet

React event objects implement some of the standard Event properties:

  • bubbles: Totuusarvo. Returns whether the event bubbles through the DOM.
  • cancelable: Totuusarvo. Returns whether the event can be canceled.
  • currentTarget: A DOM node. Returns the node to which the current handler is attached in the React tree.
  • defaultPrevented: Totuusarvo. Returns whether preventDefault was called.
  • eventPhase: Numero. Returns which phase the event is currently in.
  • isTrusted: Totuusarvo. Returns whether the event was initiated by user.
  • target: A DOM node. Returns the node on which the event has occurred (which could be a distant child).
  • timeStamp: Numero. Returns the time when the event occurred.

Additionally, React event objects provide these properties:

  • nativeEvent: A DOM Event. The original browser event object.

Metodit

React event objects implement some of the standard Event methods:

Additionally, React event objects provide these methods:

  • isDefaultPrevented(): Returns a boolean value indicating whether preventDefault was called.
  • isPropagationStopped(): Returns a boolean value indicating whether stopPropagation was called.
  • persist(): Not used with React DOM. With React Native, call this to read event’s properties after the event.
  • isPersistent(): Not used with React DOM. With React Native, returns whether persist has been called.

Rajoitukset

  • The values of currentTarget, eventPhase, target, and type reflect the values your React code expects. Under the hood, React attaches event handlers at the root, but this is not reflected in React event objects. For example, e.currentTarget may not be the same as the underlying e.nativeEvent.currentTarget. For polyfilled events, e.type (React event type) may differ from e.nativeEvent.type (underlying type).

AnimationEvent käsittelijäfunktio

An event handler type for the CSS animation events.

<div
  onAnimationStart={e => console.log('onAnimationStart')}
  onAnimationIteration={e => console.log('onAnimationIteration')}
  onAnimationEnd={e => console.log('onAnimationEnd')}
/>

Parametrit

ClipboardEvent käsittelijäfunktio

An event handler type for the Clipboard API events.

<input
  onCopy={e => console.log('onCopy')}
  onCut={e => console.log('onCut')}
  onPaste={e => console.log('onPaste')}
/>

Parametrit

CompositionEvent käsittelijäfunktio

An event handler type for the input method editor (IME) events.

<input
  onCompositionStart={e => console.log('onCompositionStart')}
  onCompositionUpdate={e => console.log('onCompositionUpdate')}
  onCompositionEnd={e => console.log('onCompositionEnd')}
/>

Parametrit

DragEvent käsittelijäfunktio

An event handler type for the HTML Drag and Drop API events.

<>
  <div
    draggable={true}
    onDragStart={e => console.log('onDragStart')}
    onDragEnd={e => console.log('onDragEnd')}
  >
    Drag source
  </div>

  <div
    onDragEnter={e => console.log('onDragEnter')}
    onDragLeave={e => console.log('onDragLeave')}
    onDragOver={e => { e.preventDefault(); console.log('onDragOver'); }}
    onDrop={e => console.log('onDrop')}
  >
    Drop target
  </div>
</>

Parametrit

FocusEvent käsittelijäfunktio

An event handler type for the focus events.

<input
  onFocus={e => console.log('onFocus')}
  onBlur={e => console.log('onBlur')}
/>

See an example.

Parametrit

Event käsittelijäfunktio

An event handler type for generic events.

Parametrit

InputEvent käsittelijäfunktio

An event handler type for the onBeforeInput event.

<input onBeforeInput={e => console.log('onBeforeInput')} />

Parametrit

KeyboardEvent käsittelijäfunktio

An event handler type for keyboard events.

<input
  onKeyDown={e => console.log('onKeyDown')}
  onKeyUp={e => console.log('onKeyUp')}
/>

See an example.

Parametrit

MouseEvent käsittelijäfunktio

An event handler type for mouse events.

<div
  onClick={e => console.log('onClick')}
  onMouseEnter={e => console.log('onMouseEnter')}
  onMouseOver={e => console.log('onMouseOver')}
  onMouseDown={e => console.log('onMouseDown')}
  onMouseUp={e => console.log('onMouseUp')}
  onMouseLeave={e => console.log('onMouseLeave')}
/>

See an example.

Parametrit

PointerEvent käsittelijäfunktio

An event handler type for pointer events.

<div
  onPointerEnter={e => console.log('onPointerEnter')}
  onPointerMove={e => console.log('onPointerMove')}
  onPointerDown={e => console.log('onPointerDown')}
  onPointerUp={e => console.log('onPointerUp')}
  onPointerLeave={e => console.log('onPointerLeave')}
/>

See an example.

Parametrit

TouchEvent käsittelijäfunktio

An event handler type for touch events.

<div
  onTouchStart={e => console.log('onTouchStart')}
  onTouchMove={e => console.log('onTouchMove')}
  onTouchEnd={e => console.log('onTouchEnd')}
  onTouchCancel={e => console.log('onTouchCancel')}
/>

Parametrit

TransitionEvent käsittelijäfunktio

An event handler type for the CSS transition events.

<div
  onTransitionEnd={e => console.log('onTransitionEnd')}
/>

Parametrit

UIEvent käsittelijäfunktio

An event handler type for generic UI events.

<div
  onScroll={e => console.log('onScroll')}
/>

Parametrit

WheelEvent käsittelijäfunktio

An event handler type for the onWheel event.

<div
  onScroll={e => console.log('onScroll')}
/>

Parametrit

Käyttö

CSS tyylien käyttö

In React, you specify a CSS class with className. It works like the class attribute in HTML:

<img className="avatar" />

Then you write the CSS rules for it in a separate CSS file:

/* In your CSS */
.avatar {
  border-radius: 50%;
}

React does not prescribe how you add CSS files. In the simplest case, you’ll add a <link> tag to your HTML. If you use a build tool or a framework, consult its documentation to learn how to add a CSS file to your project.

Sometimes, the style values depend on data. Use the style attribute to pass some styles dynamically:

<img
  className="avatar"
  style={{
    width: user.imageSize,
    height: user.imageSize
  }}
/>

In the above example, style={{}} is not a special syntax, but a regular {} object inside the style={ } JSX curly braces. We recommend only using the style attribute when your styles depend on JavaScript variables.

export default function Avatar({ user }) {
  return (
    <img
      src={user.imageUrl}
      alt={'Photo of ' + user.name}
      className="avatar"
      style={{
        width: user.imageSize,
        height: user.imageSize
      }}
    />
  );
}
Syväsukellus

Miten käyttää useita CSS-luokkia ehdollisesti?

To apply CSS classes conditionally, you need to produce the className string yourself using JavaScript.

For example, className={'row ' + (isSelected ? 'selected': '')} will produce either className="row" or className="row selected" depending on whether isSelected is true.

To make this more readable, you can use a tiny helper library like classnames:

import cn from 'classnames';

function Row({ isSelected }) {
  return (
    <div className={cn('row', isSelected && 'selected')}>
      ...
    </div>
  );
}

It is especially convenient if you have multiple conditional classes:

import cn from 'classnames';

function Row({ isSelected, size }) {
  return (
    <div className={cn('row', {
      selected: isSelected,
      large: size === 'large',
      small: size === 'small',
    })}>
      ...
    </div>
  );
}

DOM elemetin manipuolinti refillä

Sometimes, you’ll need to get the browser DOM node associated with a tag in JSX. For example, if you want to focus an <input> when a button is clicked, you need to call focus() on the browser <input> DOM node.

To obtain the browser DOM node for a tag, declare a ref and pass it as the ref attribute to that tag:

import { useRef } from 'react';

export default function Form() {
  const inputRef = useRef(null);
  // ...
  return (
    <input ref={inputRef} />
    // ...

React will put the DOM node into inputRef.current after it’s been rendered to the screen.

import { useRef } from 'react';

export default function Form() {
  const inputRef = useRef(null);

  function handleClick() {
    inputRef.current.focus();
  }

  return (
    <>
      <input ref={inputRef} />
      <button onClick={handleClick}>
        Focus the input
      </button>
    </>
  );
}

Read more about manipulating DOM with refs and check out more examples.

For more advanced use cases, the ref attribute also accepts a callback function.

Sisäisen HTML sisällön asettaminen vaarallisesti

You can pass a raw HTML string to an element like so:

const markup = { __html: '<p>some raw html</p>' };
return <div dangerouslySetInnerHTML={markup} />;

This is dangerous. As with the underlying DOM innerHTML property, you must exercise extreme caution! Unless the markup is coming from a completely trusted source, it is trivial to introduce an XSS vulnerability this way.

For example, if you use a Markdown library that converts Markdown to HTML, you trust that its parser doesn’t contain bugs, and the user only sees their own input, you can display the resulting HTML like this:

import { Remarkable } from 'remarkable';

const md = new Remarkable();

function renderMarkdownToHTML(markdown) {
  // This is ONLY safe because the output HTML
  // is shown to the same user, and because you
  // trust this Markdown parser to not have bugs.
  const renderedHTML = md.render(markdown);
  return {__html: renderedHTML};
}

export default function MarkdownPreview({ markdown }) {
  const markup = renderMarkdownToHTML(markdown);
  return <div dangerouslySetInnerHTML={markup} />;
}

To see why rendering arbitrary HTML is dangerous, replace the code above with this:

const post = {
  // Imagine this content is stored in the database.
  content: `<img src="" onerror='alert("you were hacked")'>`
};

export default function MarkdownPreview() {
  // 🔴 SECURITY HOLE: passing untrusted input to dangerouslySetInnerHTML
  const markup = { __html: post.content };
  return <div dangerouslySetInnerHTML={markup} />;
}

The code embedded in the HTML will run. A hacker could use this security hole to steal user information or to perform actions on their behalf. Only use dangerouslySetInnerHTML with trusted and sanitized data.

Hiiren tapahtumien käsitteleminen

This example shows some common mouse events and when they fire.

export default function MouseExample() {
  return (
    <div
      onMouseEnter={e => console.log('onMouseEnter (parent)')}
      onMouseLeave={e => console.log('onMouseLeave (parent)')}
    >
      <button
        onClick={e => console.log('onClick (first button)')}
        onMouseDown={e => console.log('onMouseDown (first button)')}
        onMouseEnter={e => console.log('onMouseEnter (first button)')}
        onMouseLeave={e => console.log('onMouseLeave (first button)')}
        onMouseOver={e => console.log('onMouseOver (first button)')}
        onMouseUp={e => console.log('onMouseUp (first button)')}
      >
        First button
      </button>
      <button
        onClick={e => console.log('onClick (second button)')}
        onMouseDown={e => console.log('onMouseDown (second button)')}
        onMouseEnter={e => console.log('onMouseEnter (second button)')}
        onMouseLeave={e => console.log('onMouseLeave (second button)')}
        onMouseOver={e => console.log('onMouseOver (second button)')}
        onMouseUp={e => console.log('onMouseUp (second button)')}
      >
        Second button
      </button>
    </div>
  );
}

Osoittimen tapahtumien käsitteleminen

This example shows some common pointer events and when they fire.

export default function PointerExample() {
  return (
    <div
      onPointerEnter={e => console.log('onPointerEnter (parent)')}
      onPointerLeave={e => console.log('onPointerLeave (parent)')}
      style={{ padding: 20, backgroundColor: '#ddd' }}
    >
      <div
        onPointerDown={e => console.log('onPointerDown (first child)')}
        onPointerEnter={e => console.log('onPointerEnter (first child)')}
        onPointerLeave={e => console.log('onPointerLeave (first child)')}
        onPointerMove={e => console.log('onPointerMove (first child)')}
        onPointerUp={e => console.log('onPointerUp (first child)')}
        style={{ padding: 20, backgroundColor: 'lightyellow' }}
      >
        First child
      </div>
      <div
        onPointerDown={e => console.log('onPointerDown (second child)')}
        onPointerEnter={e => console.log('onPointerEnter (second child)')}
        onPointerLeave={e => console.log('onPointerLeave (second child)')}
        onPointerMove={e => console.log('onPointerMove (second child)')}
        onPointerUp={e => console.log('onPointerUp (second child)')}
        style={{ padding: 20, backgroundColor: 'lightblue' }}
      >
        Second child
      </div>
    </div>
  );
}

Kohdennustapahtumien käsitteleminen

In React, focus events bubble. You can use the currentTarget and relatedTarget to differentiate if the focusing or blurring events originated from outside of the parent element. The example shows how to detect focusing a child, focusing the parent element, and how to detect focus entering or leaving the whole subtree.

export default function FocusExample() {
  return (
    <div
      tabIndex={1}
      onFocus={(e) => {
        if (e.currentTarget === e.target) {
          console.log('focused parent');
        } else {
          console.log('focused child', e.target.name);
        }
        if (!e.currentTarget.contains(e.relatedTarget)) {
          // Not triggered when swapping focus between children
          console.log('focus entered parent');
        }
      }}
      onBlur={(e) => {
        if (e.currentTarget === e.target) {
          console.log('unfocused parent');
        } else {
          console.log('unfocused child', e.target.name);
        }
        if (!e.currentTarget.contains(e.relatedTarget)) {
          // Not triggered when swapping focus between children
          console.log('focus left parent');
        }
      }}
    >
      <label>
        First name:
        <input name="firstName" />
      </label>
      <label>
        Last name:
        <input name="lastName" />
      </label>
    </div>
  );
}

Näppäimistötapahtumien käsitteleminen

This example shows some common keyboard events and when they fire.

export default function KeyboardExample() {
  return (
    <label>
      First name:
      <input
        name="firstName"
        onKeyDown={e => console.log('onKeyDown:', e.key, e.code)}
        onKeyUp={e => console.log('onKeyUp:', e.key, e.code)}
      />
    </label>
  );
}
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