112 lines
4.1 KiB
Markdown
112 lines
4.1 KiB
Markdown
---
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id: 5a24c314108439a4d403618b
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title: Give Sibling Elements a Unique Key Attribute
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challengeType: 6
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isRequired: false
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---
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## Description
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<section id='description'>
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The last challenge showed how the <code>map</code> method is used to dynamically render a number of elements based on user input. However, there was an important piece missing from that example. When you create an array of elements, each one needs a <code>key</code> attribute set to a unique value. React uses these keys to keep track of which items are added, changed, or removed. This helps make the re-rendering process more efficient when the list is modified in any way. Note that keys only need to be unique between sibling elements, they don't need to be globally unique in your application.
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</section>
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## Instructions
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<section id='instructions'>
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The code editor has an array with some front end frameworks and a stateless functional component named <code>Frameworks()</code>. <code>Frameworks()</code> needs to map the array to an unordered list, much like in the last challenge. Finish writing the <code>map</code> callback to return an <code>li</code> element for each framework in the <code>frontEndFrameworks</code> array. This time, make sure to give each <code>li</code> a <code>key</code> attribute, set to a unique value.
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Normally, you want to make the key something that uniquely identifies the element being rendered. As a last resort the array index may be used, but typically you should try to use a unique identification.
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: The <code>Frameworks</code> component should exist and render to the page.
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testString: assert(Enzyme.mount(React.createElement(Frameworks)).find('Frameworks').length === 1, 'The <code>Frameworks</code> component should exist and render to the page.');
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- text: <code>Frameworks</code> should render an <code>h1</code> element.
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testString: assert(Enzyme.mount(React.createElement(Frameworks)).find('h1').length === 1, '<code>Frameworks</code> should render an <code>h1</code> element.');
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- text: <code>Frameworks</code> should render a <code>ul</code> element.
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testString: assert(Enzyme.mount(React.createElement(Frameworks)).find('ul').length === 1, '<code>Frameworks</code> should render a <code>ul</code> element.');
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- text: The <code>ul</code> tag should render 6 child <code>li</code> elements.
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testString: assert(Enzyme.mount(React.createElement(Frameworks)).find('ul').children().length === 6 && Enzyme.mount(React.createElement(Frameworks)).find('ul').childAt(0).name() === 'li' && Enzyme.mount(React.createElement(Frameworks)).find('li').length === 6, 'The <code>ul</code> tag should render 6 child <code>li</code> elements.');
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- text: Each list item element should have a unique <code>key</code> attribute.
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testString: assert((() => { const ul = Enzyme.mount(React.createElement(Frameworks)).find('ul'); const keys = new Set([ ul.childAt(0).key(), ul.childAt(1).key(), ul.childAt(2).key(), ul.childAt(3).key(), ul.childAt(4).key(), ul.childAt(5).key(), ]); return keys.size === 6; })(), 'Each list item element should have a unique <code>key</code> attribute.');
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='jsx-seed'>
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```jsx
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const frontEndFrameworks = [
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'React',
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'Angular',
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'Ember',
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'Knockout',
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'Backbone',
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'Vue'
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];
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function Frameworks() {
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const renderFrameworks = null; // change code here
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return (
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<div>
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<h1>Popular Front End JavaScript Frameworks</h1>
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<ul>
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{renderFrameworks}
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</ul>
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</div>
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);
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};
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```
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</div>
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### After Test
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<div id='jsx-teardown'>
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```js
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ReactDOM.render(<Frameworks />, document.getElementById('root'))
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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const frontEndFrameworks = [
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'React',
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'Angular',
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'Ember',
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'Knockout',
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'Backbone',
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'Vue'
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];
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function Frameworks() {
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const renderFrameworks = frontEndFrameworks.map((fw, i) => {
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return <li key={i}>{fw}</li>
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})
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return (
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<div>
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<h1>Popular Front End JavaScript Frameworks</h1>
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<ul>
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{renderFrameworks}
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</ul>
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</div>
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);
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};
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```
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</section>
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