一篇文章讲清楚 React Element、React Component、JSX、Fiber 以及Diff算法的概念及其相互关系

// <div classname="snail-run">snailRun</div>;   // JSX 转化为 React ELement 的对象如下：   {     '$$typeof': Symbol(react.transitional.element),     type: 'div',     key: null,     props: { classname: 'snail-run', children: 'snailRun' },     _owner: null,     _store: {}   }

// 函数组件   function AppFunc() {     // 可以使用 react hooks     return <div>snailRun</div>;   }   // 类组件   class AppClass extends React.Component {     render() {       return <div>snailRun</div>;     }   }

<div classname="name">snailRun</div>

/**
 * Create and return a new ReactElement of the given type.
 * See https://reactjs.org/docs/react-api.html#createelement
 */
export function createElement(type, config, children) {
  if (__DEV__) {
    if (!isValidElementType(type)) {
      // This is an invalid element type.
      //
      // We warn in this case but don't throw. We expect the element creation to
      // succeed and there will likely be errors in render.
      let info = '';
      if (
        type === undefined ||
        (typeof type === 'object' &&
          type !== null &&
          Object.keys(type).length === 0)
      ) {
        info +=
          ' You likely forgot to export your component from the file ' +
          "it's defined in, or you might have mixed up default and named imports.";
      }

      let typeString;
      if (type === null) {
        typeString = 'null';
      } else if (isArray(type)) {
        typeString = 'array';
      } else if (type !== undefined && type.$$typeof === REACT_ELEMENT_TYPE) {
        typeString = `<${getComponentNameFromType(type.type) || 'Unknown'} />`;
        info =
          ' Did you accidentally export a JSX literal instead of a component?';
      } else {
        typeString = typeof type;
      }

      console.error(
        'React.createElement: type is invalid -- expected a string (for ' +
          'built-in components) or a class/function (for composite ' +
          'components) but got: %s.%s',
        typeString,
        info
      );
    } else {
      // This is a valid element type.

      // Skip key warning if the type isn't valid since our key validation logic
      // doesn't expect a non-string/function type and can throw confusing
      // errors. We don't want exception behavior to differ between dev and
      // prod. (Rendering will throw with a helpful message and as soon as the
      // type is fixed, the key warnings will appear.)
      for (let i = 2; i < arguments.length; i++) {
        validateChildKeys(arguments[i], type);
      }
    }

    // Unlike the jsx() runtime, createElement() doesn't warn about key spread.
  }

  let propName;

  // Reserved names are extracted
  const props = {};

  let key = null;
  let ref = null;

  if (config != null) {
    if (__DEV__) {
      if (
        !didWarnAboutOldJSXRuntime &&
        '__self' in config &&
        // Do not assume this is the result of an oudated JSX transform if key
        // is present, because the modern JSX transform sometimes outputs
        // createElement to preserve precedence between a static key and a
        // spread key. To avoid false positive warnings, we never warn if
        // there's a key.
        !('key' in config)
      ) {
        didWarnAboutOldJSXRuntime = true;
        console.warn(
          'Your app (or one of its dependencies) is using an outdated JSX ' +
            'transform. Update to the modern JSX transform for ' +
            'faster performance: https://react.dev/link/new-jsx-transform'
        );
      }
    }

    if (hasValidRef(config)) {
      if (!enableRefAsProp) {
        ref = config.ref;
        if (!disableStringRefs) {
          ref = coerceStringRef(ref, getOwner(), type);
        }
      }

      if (__DEV__ && !disableStringRefs) {
        warnIfStringRefCannotBeAutoConverted(config, config.__self);
      }
    }
    if (hasValidKey(config)) {
      if (__DEV__) {
        checkKeyStringCoercion(config.key);
      }
      key = '' + config.key;
    }

    // Remaining properties are added to a new props object
    for (propName in config) {
      if (
        hasOwnProperty.call(config, propName) &&
        // Skip over reserved prop names
        propName !== 'key' &&
        (enableRefAsProp || propName !== 'ref') &&
        // Even though we don't use these anymore in the runtime, we don't want
        // them to appear as props, so in createElement we filter them out.
        // We don't have to do this in the jsx() runtime because the jsx()
        // transform never passed these as props; it used separate arguments.
        propName !== '__self' &&
        propName !== '__source'
      ) {
        if (enableRefAsProp && !disableStringRefs && propName === 'ref') {
          props.ref = coerceStringRef(config[propName], getOwner(), type);
        } else {
          props[propName] = config[propName];
        }
      }
    }
  }

  // Children can be more than one argument, and those are transferred onto
  // the newly allocated props object.
  const childrenLength = arguments.length - 2;
  if (childrenLength === 1) {
    props.children = children;
  } else if (childrenLength > 1) {
    const childArray = Array(childrenLength);
    for (let i = 0; i < childrenLength; i++) {
      childArray[i] = arguments[i + 2];
    }
    if (__DEV__) {
      if (Object.freeze) {
        Object.freeze(childArray);
      }
    }
    props.children = childArray;
  }

  // Resolve default props
  if (type && type.defaultProps) {
    const defaultProps = type.defaultProps;
    for (propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }
  if (__DEV__) {
    if (key || (!enableRefAsProp && ref)) {
      const displayName =
        typeof type === 'function'
          ? type.displayName || type.name || 'Unknown'
          : type;
      if (key) {
        defineKeyPropWarningGetter(props, displayName);
      }
      if (!enableRefAsProp && ref) {
        defineRefPropWarningGetter(props, displayName);
      }
    }
  }

  const element = ReactElement(
    type,
    key,
    ref,
    undefined,
    undefined,
    getOwner(),
    props
  );

  if (type === REACT_FRAGMENT_TYPE) {
    validateFragmentProps(element);
  }

  return element;
}