142 lines
4.2 KiB
Markdown
142 lines
4.2 KiB
Markdown
---
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categories:
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- Programming Languages
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tags:
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- javascript
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- react
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- react-hooks
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---
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# Iterating through data
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As React follows the **functional programming paradigm** and state versioning,
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looping is not the recommended way of engineering repeated processes since it
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presupposes pre-existing state. Instead React relies on the functional methods
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of modern JavaScript such as `map` , `filter` , `reduce` to output the values of
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iterative data structures.
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Instead of mutating the state of a reference type, these methods are
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'non-destructive': they create a new array that meets the requirements set in
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the particular method. These newly created arrays and objects therefore form the
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basis of the state versioning, because each new array that is engendered, is a
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new state configuration.
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## The `key` attribute
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The `key` attribute is a special React attribute that should be added to the
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elements of any listed content (i.e numbered lists, unordered lists, `select`
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options etc; basically anything rendered that comes about through mapping arrays
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and objects).
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This enables React to monitor the state of the list: which items have changed,
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been added, or removed.
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The best way to pick a key is to use a string that uniquely identifies a list
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item among its siblings.
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For instance, using our earlier `numbers` array, assuming each number is unique,
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we could use the number itself as the key:
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```jsx
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const numbers = [1, 2, 3, 4, 5];
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const listItems = numbers.map((number) => (
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<li key={number.toString()}> {number}</li>
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));
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```
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> Keys have to be unique to the individual element and not shared by any other
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> sibling. But they do not need to be globally unique. You could use the same
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> `key` name in another array list.
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> Keys should be string types, not other data types. You can use the
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> `.toString()` method to ensure this is the case.
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If you can't find a unique basis for naming a series of keys, you can use the
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`index` value of the `Array.map` function. This should only be used if the order
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of the items doesn't change. An example:
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```jsx
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class List extends React.Component {
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render() {
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const demoArr = ["cat", "dog", "mouse"];
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return (
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<ul>
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{demoArr.map((x, index) => (
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<li key={index}>{x}</li>
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))}
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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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```
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## Demonstration
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### Source data
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```jsx
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const blogData = [
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{
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id: 1,
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date: "2020-11-15",
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title: "Nulla quis sem at nibh elementum imperdiet. Duis sagittis ipsum.",
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shortByline:
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"Quisque cursus, metus vitae pharetra auctor, sem massa mattis sem, at interdum magna augue eget diam. ",
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longByline:
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"Quisque cursus, metus vitae pharetra auctor, sem massa mattis sem, at interdum magna augue eget diam. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae; Morbi lacinia molestie dui. Praesent blandit dolor.",
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image: blogImage,
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},
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{
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id: 2,
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date: "2020-08-11",
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title: "Nulla quis sem at nibh elementum imperdiet. Duis sagittis ipsum.",
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shortByline:
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"Quisque cursus, metus vitae pharetra auctor, sem massa mattis sem, at interdum magna augue eget diam. ",
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longByline:
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"Quisque cursus, metus vitae pharetra auctor, sem massa mattis sem, at interdum magna augue eget diam. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae; Morbi lacinia molestie dui. Praesent blandit dolor.",
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image: blogImage,
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},
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];
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```
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### Child component
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```jsx
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export default function BlogPreviewCard(props) {
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return (
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<Card>
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<CardMedia image={props.image} />
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<CardContent>
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<Typography>{props.date}</Typography>
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<Typography>{props.title}</Typography>
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<Typography>{props.shortByline}</Typography>
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</CardContent>
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<CardActions>
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<Button>Read</Button>
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</CardActions>
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</Card>
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);
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}
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```
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### Iteration
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```jsx
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// This uses the Material-UI grid component
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import BlogPreviewCard from "../components/BlogPreviewCard"
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import blogData from "../data/blogData
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export default function BlogPreviewSet() {
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return (
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<Grid>
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{blogDummyData.map((data, index) => (
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<Grid key={index}>
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<BlogPreviewCard key={index} {...data} />
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</Grid>
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))}
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</Grid>
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)
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}
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```
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