335 lines
8.0 KiB
Markdown
335 lines
8.0 KiB
Markdown
---
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id: 5a24c314108439a4d4036183
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title: 在 React Render 方法中使用 JavaScript
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challengeType: 6
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forumTopicId: 301415
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dashedName: use-advanced-javascript-in-react-render-method
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---
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# --description--
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在之前的挑战中，你学习了如何使用大括号 `{ }` 将 JavaScript 代码插入到 JSX 代码中，用于访问 props、传递 props、访问 state、在代码中插入注释以及最近学习的定制组件样式等任务。 这些都是将 JavaScript 放在 JSX 中的常见用例，但是在 React 组件中使用 JavaScript 代码还有其他方式。
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在 `render` 方法中编写 JavaScript，可以把 JavaScript 直接放在 `return` 语句之前，而***不必***将其插入大括号中。 这是因为它还不在 JSX 代码中。 如果之后想在 `return` 语句中的 JSX 代码*里面*使用变量时，可以将变量名放在大括号中。
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# --instructions--
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在提供的代码中，`render` 方法中有一个包含 20 个短语的数组，用于表示 20 世纪 80 年代经典魔术八球玩具中的答案。 绑定 `ask` 方法到按钮的单击事件，每次单击该按钮时，将生成随机数并将其存储为 state 中的 `randomIndex`。 在第 52 行，删除字符串 `change me!` 并重新分配 `answer` 常量，以便每次组件更新时，代码随机访问 `possibleAnswers` 数组的不同值。 最后，在 `p` 标签内插入 `answer` 常量。
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# --hints--
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`MagicEightBall` 组件应该存在并被渲染到页面。
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```js
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assert.strictEqual(
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Enzyme.mount(React.createElement(MagicEightBall)).find('MagicEightBall')
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.length,
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1
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);
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```
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`MagicEightBall` 的第一个子元素应该是 `input` 元素。
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```js
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assert.strictEqual(
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Enzyme.mount(React.createElement(MagicEightBall))
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.children()
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.childAt(0)
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.name(),
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'input'
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);
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```
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`MagicEightBall` 的第三个子元素应该是 `button` 元素。
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```js
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assert.strictEqual(
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Enzyme.mount(React.createElement(MagicEightBall))
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.children()
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.childAt(2)
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.name(),
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'button'
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);
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```
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`MagicEightBall` 的 state 应该用 `userInput` 属性和 `randomIndex` 属性初始化，并且这两个属性的值都应该是空字符串。
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```js
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assert(
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Enzyme.mount(React.createElement(MagicEightBall)).state('randomIndex') ===
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'' &&
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Enzyme.mount(React.createElement(MagicEightBall)).state('userInput') === ''
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);
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```
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当 `MagicEightBall` 第一次加载到 DOM 中时，它应该返回一个空的 `p` 元素。
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```js
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assert(
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Enzyme.mount(React.createElement(MagicEightBall)).find('p').length === 1 &&
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Enzyme.mount(React.createElement(MagicEightBall)).find('p').text() === ''
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);
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```
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当文本被输入到 `input` 元素中并点击按钮时， `MagicEightBall` 组件应该返回一个 `p` 元素，该元素包含数组 `possibleAnswers` 中的随机一个元素。
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```js
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(() => {
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const comp = Enzyme.mount(React.createElement(MagicEightBall));
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const simulate = () => {
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comp.find('input').simulate('change', { target: { value: 'test?' } });
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comp.find('button').simulate('click');
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};
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const result = () => comp.find('p').text();
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const _1 = () => {
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simulate();
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return result();
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};
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const _2 = () => {
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simulate();
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return result();
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};
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const _3 = () => {
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simulate();
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return result();
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};
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const _4 = () => {
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simulate();
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return result();
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};
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const _5 = () => {
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simulate();
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return result();
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};
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const _6 = () => {
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simulate();
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return result();
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};
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const _7 = () => {
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simulate();
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return result();
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};
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const _8 = () => {
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simulate();
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return result();
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};
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const _9 = () => {
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simulate();
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return result();
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};
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const _10 = () => {
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simulate();
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return result();
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};
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const _1_val = _1();
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const _2_val = _2();
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const _3_val = _3();
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const _4_val = _4();
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const _5_val = _5();
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const _6_val = _6();
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const _7_val = _7();
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const _8_val = _8();
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const _9_val = _9();
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const _10_val = _10();
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const actualAnswers = [
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_1_val,
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_2_val,
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_3_val,
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_4_val,
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_5_val,
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_6_val,
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_7_val,
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_8_val,
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_9_val,
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_10_val
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];
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const hasIndex = actualAnswers.filter(
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(answer, i) => possibleAnswers.indexOf(answer) !== -1
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);
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const notAllEqual = new Set(actualAnswers);
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assert(notAllEqual.size > 1 && hasIndex.length === 10);
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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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var possibleAnswers = [
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'It is certain',
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'It is decidedly so',
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'Without a doubt',
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'Yes, definitely',
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'You may rely on it',
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'As I see it, yes',
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'Outlook good',
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'Yes',
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'Signs point to yes',
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'Reply hazy try again',
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'Ask again later',
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'Better not tell you now',
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'Cannot predict now',
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'Concentrate and ask again',
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"Don't count on it",
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'My reply is no',
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'My sources say no',
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'Outlook not so good',
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'Very doubtful',
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'Most likely'
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];
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ReactDOM.render(<MagicEightBall />, document.getElementById('root'));
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```
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## --seed-contents--
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```jsx
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const inputStyle = {
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width: 235,
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margin: 5
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};
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class MagicEightBall 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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userInput: '',
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randomIndex: ''
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};
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this.ask = this.ask.bind(this);
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this.handleChange = this.handleChange.bind(this);
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}
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ask() {
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if (this.state.userInput) {
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this.setState({
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randomIndex: Math.floor(Math.random() * 20),
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userInput: ''
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});
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}
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}
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handleChange(event) {
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this.setState({
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userInput: event.target.value
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});
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}
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render() {
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const possibleAnswers = [
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'It is certain',
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'It is decidedly so',
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'Without a doubt',
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'Yes, definitely',
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'You may rely on it',
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'As I see it, yes',
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'Outlook good',
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'Yes',
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'Signs point to yes',
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'Reply hazy try again',
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'Ask again later',
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'Better not tell you now',
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'Cannot predict now',
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'Concentrate and ask again',
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"Don't count on it",
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'My reply is no',
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'My sources say no',
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'Most likely',
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'Outlook not so good',
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'Very doubtful'
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];
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const answer = 'change me!'; // Change this line
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return (
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<div>
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<input
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type='text'
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value={this.state.userInput}
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onChange={this.handleChange}
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style={inputStyle}
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/>
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<br />
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<button onClick={this.ask}>Ask the Magic Eight Ball!</button>
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<br />
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<h3>Answer:</h3>
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<p>
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{/* Change code below this line */}
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{/* Change code above this line */}
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</p>
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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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const inputStyle = {
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width: 235,
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margin: 5
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};
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class MagicEightBall 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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userInput: '',
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randomIndex: ''
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};
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this.ask = this.ask.bind(this);
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this.handleChange = this.handleChange.bind(this);
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}
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ask() {
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if (this.state.userInput) {
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this.setState({
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randomIndex: Math.floor(Math.random() * 20),
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userInput: ''
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});
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}
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}
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handleChange(event) {
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this.setState({
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userInput: event.target.value
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});
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}
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render() {
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const possibleAnswers = [
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'It is certain',
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'It is decidedly so',
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'Without a doubt',
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'Yes, definitely',
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'You may rely on it',
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'As I see it, yes',
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'Outlook good',
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'Yes',
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'Signs point to yes',
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'Reply hazy try again',
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'Ask again later',
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'Better not tell you now',
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'Cannot predict now',
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'Concentrate and ask again',
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"Don't count on it",
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'My reply is no',
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'My sources say no',
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'Outlook not so good',
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'Very doubtful',
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'Most likely'
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];
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const answer = possibleAnswers[this.state.randomIndex];
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return (
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<div>
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<input
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type='text'
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value={this.state.userInput}
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onChange={this.handleChange}
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style={inputStyle}
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/>
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<br />
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<button onClick={this.ask}>Ask the Magic Eight Ball!</button>
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<br />
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<h3>Answer:</h3>
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<p>{answer}</p>
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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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