freeCodeCamp/curriculum/challenges/chinese/03-front-end-libraries/react/use-state-to-toggle-an-element.md

---
id: 5a24c314108439a4d4036176
title: Use State to Toggle an Element
challengeType: 6
forumTopicId: 301421
dashedName: use-state-to-toggle-an-element
---

# --description--

Sometimes you might need to know the previous state when updating the state. However, state updates may be asynchronous - this means React may batch multiple `setState()` calls into a single update. This means you can't rely on the previous value of `this.state` or `this.props` when calculating the next value. So, you should not use code like this:

```jsx
this.setState({
  counter: this.state.counter + this.props.increment
});
```

Instead, you should pass `setState` a function that allows you to access state and props. Using a function with `setState` guarantees you are working with the most current values of state and props. This means that the above should be rewritten as:

```jsx
this.setState((state, props) => ({
  counter: state.counter + props.increment
}));
```

You can also use a form without `props` if you need only the `state`:

```jsx
this.setState(state => ({
  counter: state.counter + 1
}));
```

Note that you have to wrap the object literal in parentheses, otherwise JavaScript thinks it's a block of code.

# --instructions--

`MyComponent` has a `visibility` property which is initialized to `false`. The render method returns one view if the value of `visibility` is true, and a different view if it is false.

Currently, there is no way of updating the `visibility` property in the component's `state`. The value should toggle back and forth between true and false. There is a click handler on the button which triggers a class method called `toggleVisibility()`. Pass a function to `setState` to define this method so that the `state` of `visibility` toggles to the opposite value when the method is called. If `visibility` is `false`, the method sets it to `true`, and vice versa.

Finally, click the button to see the conditional rendering of the component based on its `state`.

**Hint:** Don't forget to bind the `this` keyword to the method in the constructor!

# --hints--

`MyComponent` should return a `div` element which contains a `button`.

```js
assert.strictEqual(
  Enzyme.mount(React.createElement(MyComponent)).find('div').find('button')
    .length,
  1
);
```

The state of `MyComponent` should initialize with a `visibility` property set to `false`.

```js
assert.strictEqual(
  Enzyme.mount(React.createElement(MyComponent)).state('visibility'),
  false
);
```

Clicking the button element should toggle the `visibility` property in state between `true` and `false`.

```js
(() => {
  const mockedComponent = Enzyme.mount(React.createElement(MyComponent));
  const first = () => {
    mockedComponent.setState({ visibility: false });
    return mockedComponent.state('visibility');
  };
  const second = () => {
    mockedComponent.find('button').simulate('click');
    return mockedComponent.state('visibility');
  };
  const third = () => {
    mockedComponent.find('button').simulate('click');
    return mockedComponent.state('visibility');
  };
  const firstValue = first();
  const secondValue = second();
  const thirdValue = third();
  assert(!firstValue && secondValue && !thirdValue);
})();
```

An anonymous function should be passed to `setState`.

```js
const paramRegex = '[a-zA-Z$_]\\w*(,[a-zA-Z$_]\\w*)?';
assert(
  new RegExp(
    'this\\.setState\\((function\\(' +
      paramRegex +
      '\\){|([a-zA-Z$_]\\w*|\\(' +
      paramRegex +
      '\\))=>)'
  ).test(__helpers.removeWhiteSpace(code))
);
```

`this` should not be used inside `setState`

```js
assert(!/this\.setState\([^}]*this/.test(code));
```

# --seed--

## --after-user-code--

```jsx
ReactDOM.render(<MyComponent />, document.getElementById('root'));
```

## --seed-contents--

```jsx
class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      visibility: false
    };
    // Change code below this line

    // Change code above this line
  }
  // Change code below this line

  // Change code above this line
  render() {
    if (this.state.visibility) {
      return (
        <div>
          <button onClick={this.toggleVisibility}>Click Me</button>
          <h1>Now you see me!</h1>
        </div>
      );
    } else {
      return (
        <div>
          <button onClick={this.toggleVisibility}>Click Me</button>
        </div>
      );
    }
  }
}
```

# --solutions--

```jsx
class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      visibility: false
    };
    this.toggleVisibility = this.toggleVisibility.bind(this);
  }
  toggleVisibility() {
    this.setState(state => ({
      visibility: !state.visibility
    }));
  }
  render() {
    if (this.state.visibility) {
      return (
        <div>
          <button onClick={this.toggleVisibility}>Click Me</button>
          <h1>Now you see me!</h1>
        </div>
      );
    } else {
      return (
        <div>
          <button onClick={this.toggleVisibility}>Click Me</button>
        </div>
      );
    }
  }
}
```