React gives you this warning because it likes that you chose between controlled and uncontrolled components.
In the case of a checkbox input the component is considered controlled when its checked prop is not undefined.
I've just given a default value for checked and changed the code testing for undefined a little bit.
The warning should be gone.
<!-- begin snippet: js hide: false console: true babel: true -->
<!-- language: lang-js -->
// import React, { Component } from "react";
class Checkbox extends React.Component {
static defaultProps = {
checked: false
}
render() {
return (
<input
type={this.props.type}
name={this.props.name}
checked={this.props.checked}
onChange={this.props.onChange}
/>
);
}
}
class App extends React.Component {
constructor(props) {
super(props);
this.state = {
checkedItems: new Map()
};
this.handleChange = this.handleChange.bind(this);
}
handleChange = e => {
const item = e.target.name;
const isChecked = e.target.checked;
this.setState(prevState => ({
checkedItems: prevState.checkedItems.set(item, isChecked)
}));
};
deleteCheckboxState = (name, checked) => {
const updateChecked = checked == null ? true : false;
this.setState(prevState => prevState.checkedItems.set(name, updateChecked));
};
clearAllCheckboxes = () => {
const clearCheckedItems = new Map();
this.setState({ checkedItems: clearCheckedItems });
};
render() {
const checkboxes = [
{
name: "check-box-1",
key: "checkBox1",
label: "Check Box 1"
},
{
name: "check-box-2",
key: "checkBox2",
label: "Check Box 2"
},
{
name: "check-box-3",
key: "checkBox3",
label: "Check Box 3"
},
{
name: "check-box-4",
key: "checkBox4",
label: "Check Box 4"
}
];
const checkboxesToRender = checkboxes.map(item => {
return (
<label key={item.key}>
{item.name}
<Checkbox
name={item.name}
checked={this.state.checkedItems.get(item.name)}
onChange={this.handleChange}
type="checkbox"
/>
</label>
);
});
const checkboxesDeleteHandlers = checkboxes.map(item => {
return (
<span
key={item.name}
onClick={() =>
this.deleteCheckboxState(
item.name,
this.state.checkedItems.get(item.name)
)
}
>
{item.name}
</span>
);
});
return (
<div className="App">
{checkboxesToRender}
<br /> {checkboxesDeleteHandlers}
<p onClick={this.clearAllCheckboxes}>clear all</p>
</div>
);
}
}
ReactDOM.render(<App />, document.getElementById('root'));
<!-- 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>
<div id="root"></div>
<!-- end snippet -->
That's *property spread notation*. It was added in ES2018, but long-supported in React projects via transpilation (as "JSX spread attributes" even though you could do it elsewhere, too, not just attributes).
`{...this.props}` *spreads out* the "own" 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>
<!-- end snippet -->
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"`:
<!-- begin snippet: js hide: true console: true babel: false -->
<!-- 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;
}
<!-- end snippet -->
[1]: https://reactjs.org/docs/jsx-in-depth.html#children-in-jsx
