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<!-- language: lang-js -->
import React, { useState } from "react";
import "./styles.css";
export default function App() {
const [chkArray, setChkArray] = useState([]);
const onChangeHandler = e => {
//Creating a new copy of the existing Array
const newChkArray = [...chkArray];
//checking if the checkbox selected is true.
//If yes then adding the value to the checkArray
if (e.target.checked) {
newChkArray.push(e.target.value);
} else if (newChkArray.indexOf(e.target.value) >= 0) {
//If unchecking an already selected item then removing it from the array
newChkArray.splice(newChkArray.indexOf(e.target.value), 1);
}
//Updating the list with new items checked.
setChkArray(newChkArray);
};
return (
<div className="App">
<input
type="checkbox"
id="chk1"
name="chk1"
value="chk1"
onChange={e => onChangeHandler(e)}
/>
<label htmlFor="chk1"> Chk 1</label>
<br />
<input
type="checkbox"
id="chk2"
name="chk2"
value="chk2"
onChange={e => onChangeHandler(e)}
/>
<label htmlFor="chk2"> Chk 2</label>
<br />
<input
type="checkbox"
id="chk3"
name="chk3"
value="chk3"
onChange={e => onChangeHandler(e)}
/>
<label htmlFor="chk3"> Chk 3</label>
<br />
<input
type="checkbox"
id="chk4"
name="chk4"
value="chk4"
onChange={e => onChangeHandler(e)}
/>
<label htmlFor="chk4"> Chk 4</label>
<br />
{JSON.stringify(chkArray)}
</div>
);
}
<!-- language: lang-html -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
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That's [*property spread notation*][1]. It was added in ES2018 (spread for arrays/iterables was earlier, ES2015), but it's been supported in React projects for a long time via transpilation (as "[JSX spread attributes][2]" even though you could do it elsewhere, too, not just attributes).
`{...this.props}` *spreads out* the "own" enumerable properties in `props` as discrete properties on the `Modal` element you're creating. For instance, if `this.props` contained `a: 1` and `b: 2`, then
<Modal {...this.props} title='Modal heading' animation={false}>
would be the same as
<Modal a={this.props.a} b={this.props.b} title='Modal heading' animation={false}>
But it's dynamic, so whatever "own" properties are in `props` are included.
Since `children` is an "own" property in `props`, spread will include it. So if the component where this appears had child elements, they'll be passed on to `Modal`. Putting child elements between the opening tag and closing tags is just syntactic sugar — the good kind — for putting a `children` property in the opening tag. Example:
<!-- begin snippet: js hide: true console: true babel: true -->
<!-- language: lang-js -->
class Example extends React.Component {
render() {
const { className, children } = this.props;
return (
<div className={className}>
{children}
</div>
);
}
}
ReactDOM.render(
[
<Example className="first">
<span>Child in first</span>
</Example>,
<Example className="second" children={<span>Child in second</span>} />
],
document.getElementById("root")
);
<!-- language: lang-css -->
.first {
color: green;
}
.second {
color: blue;
}
<!-- language: lang-html -->
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
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Spread notation is handy not only for that use case, but for creating a new object with most (or all) of the properties of an existing object — which comes up a lot when you're updating state, since you can't modify state directly:
this.setState(prevState => {
return {foo: {...prevState.foo, a: "updated"}};
});
That replaces `this.state.foo` with a new object with all the same properties as `foo` except the `a` property, which becomes `"updated"`:
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<!-- language: lang-js -->
const obj = {
foo: {
a: 1,
b: 2,
c: 3
}
};
console.log("original", obj.foo);
// Creates a NEW object and assigns it to `obj.foo`
obj.foo = {...obj.foo, a: "updated"};
console.log("updated", obj.foo);
<!-- language: lang-css -->
.as-console-wrapper {
max-height: 100% !important;
}
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[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax
[2]: https://reactjs.org/docs/jsx-in-depth.html#spread-attributes