148 lines
3.8 KiB
Markdown
148 lines
3.8 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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forumTopicId: 301394
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dashedName: give-sibling-elements-a-unique-key-attribute
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---
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# --description--
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The last challenge showed how the `map` 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 `key` 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.
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**Note:** Keys only need to be unique between sibling elements, they don't need to be globally unique in your application.
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# --instructions--
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The code editor has an array with some front end frameworks and a stateless functional component named `Frameworks()`. `Frameworks()` needs to map the array to an unordered list, much like in the last challenge. Finish writing the `map` callback to return an `li` element for each framework in the `frontEndFrameworks` array. This time, make sure to give each `li` a `key` attribute, set to a unique value. The `li` elements should also contain text from `frontEndFrameworks`.
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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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# --hints--
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The `Frameworks` component should exist and render to the page.
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```js
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assert(
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Enzyme.mount(React.createElement(Frameworks)).find('Frameworks').length === 1
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);
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```
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`Frameworks` should render an `h1` element.
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```js
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assert(Enzyme.mount(React.createElement(Frameworks)).find('h1').length === 1);
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```
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`Frameworks` should render a `ul` element.
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```js
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assert(Enzyme.mount(React.createElement(Frameworks)).find('ul').length === 1);
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```
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The `ul` tag should render 6 child `li` elements.
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```js
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assert(
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Enzyme.mount(React.createElement(Frameworks)).find('ul').children().length ===
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6 &&
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Enzyme.mount(React.createElement(Frameworks))
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.find('ul')
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.childAt(0)
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.name() === 'li' &&
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Enzyme.mount(React.createElement(Frameworks)).find('li').length === 6
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);
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```
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Each list item element should have a unique `key` attribute.
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```js
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assert(
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(() => {
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const ul = Enzyme.mount(React.createElement(Frameworks)).find('ul');
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const keys = new Set([
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ul.childAt(0).key(),
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ul.childAt(1).key(),
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ul.childAt(2).key(),
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ul.childAt(3).key(),
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ul.childAt(4).key(),
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ul.childAt(5).key()
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]);
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return keys.size === 6;
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})()
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);
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```
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Each list item element should contain text from `frontEndFrameworks`.
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```js
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assert(
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(() => {
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const li = Enzyme.mount(React.createElement(Frameworks))
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.find('ul')
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.children();
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return [...Array(5)].every((_, i) =>
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frontEndFrameworks.includes(li.at(i).text())
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);
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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(<Frameworks />, document.getElementById('root'))
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```
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## --seed-contents--
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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 this line
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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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# --solutions--
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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 = frontEndFrameworks.map((fw, i) => <li key={i}>{fw}</li>);
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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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