190 lines
5.0 KiB
Markdown
190 lines
5.0 KiB
Markdown
---
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id: 5a24c314108439a4d4036176
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title: Use State to Toggle an Element
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challengeType: 6
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forumTopicId: 301421
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dashedName: use-state-to-toggle-an-element
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---
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# --description--
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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:
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```jsx
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this.setState({
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counter: this.state.counter + this.props.increment
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});
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```
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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:
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```jsx
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this.setState((state, props) => ({
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counter: state.counter + props.increment
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}));
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```
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You can also use a form without `props` if you need only the `state`:
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```jsx
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this.setState(state => ({
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counter: state.counter + 1
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}));
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```
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Note that you have to wrap the object literal in parentheses, otherwise JavaScript thinks it's a block of code.
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# --instructions--
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`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.
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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.
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Finally, click the button to see the conditional rendering of the component based on its `state`.
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**Hint:** Don't forget to bind the `this` keyword to the method in the `constructor`!
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# --hints--
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`MyComponent` should return a `div` element which contains a `button`.
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```js
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assert.strictEqual(
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Enzyme.mount(React.createElement(MyComponent)).find('div').find('button')
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.length,
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1
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);
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```
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The state of `MyComponent` should initialize with a `visibility` property set to `false`.
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```js
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assert.strictEqual(
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Enzyme.mount(React.createElement(MyComponent)).state('visibility'),
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false
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);
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```
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Clicking the button element should toggle the `visibility` property in state between `true` and `false`.
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```js
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(() => {
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const mockedComponent = Enzyme.mount(React.createElement(MyComponent));
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const first = () => {
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mockedComponent.setState({ visibility: false });
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return mockedComponent.state('visibility');
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};
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const second = () => {
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mockedComponent.find('button').simulate('click');
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return mockedComponent.state('visibility');
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};
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const third = () => {
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mockedComponent.find('button').simulate('click');
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return mockedComponent.state('visibility');
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};
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const firstValue = first();
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const secondValue = second();
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const thirdValue = third();
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assert(!firstValue && secondValue && !thirdValue);
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})();
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```
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An anonymous function should be passed to `setState`.
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```js
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const paramRegex = '[a-zA-Z$_]\\w*(,[a-zA-Z$_]\\w*)?';
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assert(
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new RegExp(
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'this\\.setState\\((function\\(' +
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paramRegex +
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'\\){|([a-zA-Z$_]\\w*|\\(' +
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paramRegex +
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'\\))=>)'
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).test(__helpers.removeWhiteSpace(code))
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);
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```
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`this` should not be used inside `setState`
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```js
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assert(!/this\.setState\([^}]*this/.test(code));
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```
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# --seed--
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## --after-user-code--
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```jsx
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ReactDOM.render(<MyComponent />, document.getElementById('root'));
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```
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## --seed-contents--
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```jsx
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class MyComponent extends React.Component {
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constructor(props) {
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super(props);
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this.state = {
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visibility: false
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};
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// Change code below this line
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// Change code above this line
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}
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// Change code below this line
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// Change code above this line
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render() {
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if (this.state.visibility) {
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return (
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<div>
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<button onClick={this.toggleVisibility}>Click Me</button>
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<h1>Now you see me!</h1>
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</div>
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);
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} else {
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return (
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<div>
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<button onClick={this.toggleVisibility}>Click Me</button>
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</div>
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);
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}
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}
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}
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```
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# --solutions--
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```jsx
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class MyComponent extends React.Component {
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constructor(props) {
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super(props);
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this.state = {
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visibility: false
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};
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this.toggleVisibility = this.toggleVisibility.bind(this);
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}
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toggleVisibility() {
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this.setState(state => ({
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visibility: !state.visibility
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}));
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}
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render() {
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if (this.state.visibility) {
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return (
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<div>
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<button onClick={this.toggleVisibility}>Click Me</button>
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<h1>Now you see me!</h1>
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</div>
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);
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} else {
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return (
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<div>
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<button onClick={this.toggleVisibility}>Click Me</button>
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</div>
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);
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}
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}
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}
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```
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