144 lines
4.6 KiB
Markdown
144 lines
4.6 KiB
Markdown
---
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id: 5a24c314108439a4d4036173
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title: Set State with this.setState
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challengeType: 6
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forumTopicId: 301412
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dashedName: set-state-with-this-setstate
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---
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# --description--
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The previous challenges covered component `state` and how to initialize state in the `constructor`. There is also a way to change the component's `state`. React provides a method for updating component `state` called `setState`. You call the `setState` method within your component class like so: `this.setState()`, passing in an object with key-value pairs. The keys are your state properties and the values are the updated state data. For instance, if we were storing a `username` in state and wanted to update it, it would look like this:
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```jsx
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this.setState({
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username: 'Lewis'
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});
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```
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React expects you to never modify `state` directly, instead always use `this.setState()` when state changes occur. Also, you should note that React may batch multiple state updates in order to improve performance. What this means is that state updates through the `setState` method can be asynchronous. There is an alternative syntax for the `setState` method which provides a way around this problem. This is rarely needed but it's good to keep it in mind! Please consult the [React documentation](https://facebook.github.io/react/docs/state-and-lifecycle.html) for further details.
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# --instructions--
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There is a `button` element in the code editor which has an `onClick()` handler. This handler is triggered when the `button` receives a click event in the browser, and runs the `handleClick` method defined on `MyComponent`. Within the `handleClick` method, update the component `state` using `this.setState()`. Set the `name` property in `state` to equal the string `React Rocks!`.
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Click the button and watch the rendered state update. Don't worry if you don't fully understand how the click handler code works at this point. It's covered in upcoming challenges.
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# --hints--
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The state of `MyComponent` should initialize with the key value pair `{ name: Initial State }`.
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```js
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assert(
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Enzyme.mount(React.createElement(MyComponent)).state('name') ===
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'Initial State'
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);
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```
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`MyComponent` should render an `h1` header.
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```js
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assert(Enzyme.mount(React.createElement(MyComponent)).find('h1').length === 1);
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```
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The rendered `h1` header should contain text rendered from the component's state.
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```js
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async () => {
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const waitForIt = (fn) =>
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new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250));
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const mockedComponent = Enzyme.mount(React.createElement(MyComponent));
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const first = () => {
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mockedComponent.setState({ name: 'TestName' });
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return waitForIt(() => mockedComponent.html());
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};
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const firstValue = await first();
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assert(/<h1>TestName<\/h1>/.test(firstValue));
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};
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```
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Calling the `handleClick` method on `MyComponent` should set the name property in state to equal `React Rocks!`.
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```js
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async () => {
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const waitForIt = (fn) =>
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new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250));
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const mockedComponent = Enzyme.mount(React.createElement(MyComponent));
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const first = () => {
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mockedComponent.setState({ name: 'Before' });
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return waitForIt(() => mockedComponent.state('name'));
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};
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const second = () => {
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mockedComponent.instance().handleClick();
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return waitForIt(() => mockedComponent.state('name'));
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};
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const firstValue = await first();
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const secondValue = await second();
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assert(firstValue === 'Before' && secondValue === 'React Rocks!');
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};
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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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name: 'Initial State'
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};
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this.handleClick = this.handleClick.bind(this);
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}
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handleClick() {
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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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render() {
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return (
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<div>
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<button onClick={this.handleClick}>Click Me</button>
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<h1>{this.state.name}</h1>
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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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# --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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name: 'Initial State'
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};
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this.handleClick = this.handleClick.bind(this);
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}
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handleClick() {
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// Change code below this line
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this.setState({
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name: 'React Rocks!'
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});
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// Change code above this line
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}
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render() {
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return (
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<div>
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<button onClick = {this.handleClick}>Click Me</button>
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<h1>{this.state.name}</h1>
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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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