2018-10-10 22:03:03 +00:00
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---
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id: 5a24c314108439a4d4036185
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2021-02-06 04:42:36 +00:00
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title: Use && for a More Concise Conditional
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2018-10-10 22:03:03 +00:00
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challengeType: 6
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2020-09-17 16:13:42 +00:00
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forumTopicId: 301413
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2021-01-13 02:31:00 +00:00
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dashedName: use--for-a-more-concise-conditional
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2018-10-10 22:03:03 +00:00
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---
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2020-12-16 07:37:30 +00:00
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# --description--
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2018-10-10 22:03:03 +00:00
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2021-03-05 15:43:24 +00:00
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`if/else` 语句在上一次挑战中是有效的,但是有一种更简洁的方法可以达到同样的结果。 假设正在跟踪组件中的几个条件,并且希望根据这些条件中的每一个来渲染不同的元素。 如果你写了很多 `else if` 语句来返回稍微不同的 UI,你可能会写很多重复代码,这就留下了出错的空间。 相反,你可以使用 `&&` 逻辑运算符以更简洁的方式执行条件逻辑。 这是完全可行的,因为你希望检查条件是否为 `true`。如果是,则返回一些标记。 这里有一个例子:
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2018-10-10 22:03:03 +00:00
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2021-03-22 13:52:28 +00:00
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```jsx
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{condition && <p>markup</p>}
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```
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2020-12-16 07:37:30 +00:00
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2021-03-22 13:52:28 +00:00
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如果 `condition` 为 `true`,则返回标记。 如果条件为 `false` ,则在评估 `condition` 后操作将立即返回 `false`,并且不返回任何内容。 可以将这些语句直接包含在 JSX 中,并通过在每个条件后面写 `&&` 来将多个条件串在一起。 这允许你在 `render()` 方法中处理更复杂的条件逻辑,而无需重复大量代码。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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# --instructions--
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2018-10-10 22:03:03 +00:00
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2021-03-22 13:52:28 +00:00
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再来看看前面的示例,`h1` 还是在 `display` 为 `true` 时渲染,但使用 `&&` 逻辑运算符代替 `if/else` 语句。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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# --hints--
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2021-03-22 13:52:28 +00:00
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`MyComponent` 应该存在并渲染。
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2018-10-10 22:03:03 +00:00
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```js
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2020-12-16 07:37:30 +00:00
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assert(
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(function () {
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const mockedComponent = Enzyme.mount(React.createElement(MyComponent));
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return mockedComponent.find('MyComponent').length;
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})()
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);
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2018-10-10 22:03:03 +00:00
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```
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2021-03-22 13:52:28 +00:00
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当 `display` 被设置为 `true` 时,`div`、`button` 和 `h1` 标签应该渲染。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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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 state_1 = () => {
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mockedComponent.setState({ display: true });
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return waitForIt(() => mockedComponent);
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};
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const updated = await state_1();
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assert(
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updated.find('div').length === 1 &&
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updated.find('div').children().length === 2 &&
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updated.find('button').length === 1 &&
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updated.find('h1').length === 1
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);
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};
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```
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2018-10-10 22:03:03 +00:00
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2021-03-22 13:52:28 +00:00
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当 `display` 被设置为 `false` 时,只有 `div` 和 `button` 应该渲染。
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2020-09-17 16:13:42 +00:00
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2018-10-10 22:03:03 +00:00
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```js
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2020-12-16 07:37:30 +00:00
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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 state_1 = () => {
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mockedComponent.setState({ display: false });
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return waitForIt(() => mockedComponent);
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};
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const updated = await state_1();
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assert(
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updated.find('div').length === 1 &&
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updated.find('div').children().length === 1 &&
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updated.find('button').length === 1 &&
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updated.find('h1').length === 0
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);
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2020-09-17 16:13:42 +00:00
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};
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2018-10-10 22:03:03 +00:00
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```
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2020-08-13 15:24:35 +00:00
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2021-03-05 15:43:24 +00:00
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render 方法应该使用 `&&` 逻辑运算符来检查 `this.state.display` 的条件。
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2020-12-16 07:37:30 +00:00
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```js
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(getUserInput) => assert(getUserInput('index').includes('&&'));
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```
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2021-01-13 02:31:00 +00:00
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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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display: true
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}
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this.toggleDisplay = this.toggleDisplay.bind(this);
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}
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toggleDisplay() {
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this.setState(state => ({
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display: !state.display
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}));
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}
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render() {
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// Change code below this line
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return (
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<div>
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<button onClick={this.toggleDisplay}>Toggle Display</button>
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<h1>Displayed!</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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2020-12-16 07:37:30 +00:00
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# --solutions--
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2021-01-13 02:31:00 +00:00
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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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display: true
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}
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this.toggleDisplay = this.toggleDisplay.bind(this);
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}
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toggleDisplay() {
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this.setState(state => ({
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display: !state.display
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}));
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}
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render() {
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// Change code below this line
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return (
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<div>
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<button onClick={this.toggleDisplay}>Toggle Display</button>
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{this.state.display && <h1>Displayed!</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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