79 lines
1.9 KiB
Markdown
79 lines
1.9 KiB
Markdown
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---
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id: 5a24c314108439a4d403616b
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title: 使用默認的 Props
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challengeType: 6
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forumTopicId: 301418
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dashedName: use-default-props
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---
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# --description--
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React 還有一個設置默認 props 的選項。 可以將默認 props 作爲組件本身的屬性分配給組件，React 會在必要時分配默認 prop。 如果沒有顯式的提供任何值，這允許指定 prop 值應該是什麼。 例如，如果聲明 `MyComponent.defaultProps = { location: 'San Francisco' }`，即定義一個 location 屬性，並且其值在沒有另行制定的情況下被設置爲字符串 `San Francisco`。 如果 props 未定義，則 React 會分配默認 props，但如果你將 `null` 作爲 prop 的值，它將保持 `null`。
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# --instructions--
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代碼編輯器中有一個 `ShoppingCart` 組件。 在這個組件上定義默認 props，它指定一個 `items` prop，其值爲 `0`。
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# --hints--
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應該渲染 `ShoppingCart` 組件。
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```js
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assert(
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(function () {
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const mockedComponent = Enzyme.mount(React.createElement(ShoppingCart));
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return mockedComponent.find('ShoppingCart').length === 1;
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})()
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);
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```
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`ShoppingCart` 組件應該有一個 `{ items: 0 }` 的默認 prop。
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```js
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assert(
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(function () {
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const mockedComponent = Enzyme.mount(React.createElement(ShoppingCart));
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mockedComponent.setProps({ items: undefined });
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return mockedComponent.find('ShoppingCart').props().items === 0;
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})()
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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(<ShoppingCart />, document.getElementById('root'))
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```
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## --seed-contents--
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```jsx
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const ShoppingCart = (props) => {
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return (
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<div>
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<h1>Shopping Cart Component</h1>
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</div>
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)
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};
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// Change code below this line
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```
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# --solutions--
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```jsx
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const ShoppingCart = (props) => {
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return (
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<div>
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<h1>Shopping Cart Component</h1>
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</div>
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)
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};
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// Change code below this line
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ShoppingCart.defaultProps = {
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items: 0
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}
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```
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