I described the solution in a comment, but here's a jsfiddle with more detail:
The idea here would be to store the time in the state, and to update the state every time you want the timer to update. The place to set a `setInterval` inside stateful components is usually `componentDidMount`
https://jsfiddle.net/nfr6phw2/
<!-- begin snippet: js hide: false console: true babel: true -->
<!-- language: lang-js -->
class TodoApp extends React.Component {
constructor(props) {
super(props)
this.state = {
datetime: new Date()
}
}
componentDidMount() {
setInterval(() => {
this.setState({datetime: new Date()})
}, 1000);
}
getDateTime() {
let now = new Date();
let year = now.getFullYear();
let month = now.getMonth()+1;
let day = now.getDate();
let hour = now.getHours();
let minute = now.getMinutes();
let second = now.getSeconds();
if(month.toString().length === 1) {
month = '0'+month;
}
if(day.toString().length === 1) {
day = '0'+day;
}
if(hour.toString().length === 1) {
hour = '0'+hour;
}
if(minute.toString().length === 1) {
minute = '0'+minute;
}
if(second.toString().length === 1) {
second = '0'+second;
}
var dateTime = hour+':'+minute+':'+second;
return dateTime;
}
render() {
return (
<div>
<h2>The time is {this.getDateTime()}</h2>
</div>
)
}
}
ReactDOM.render(<TodoApp />, document.querySelector("#app"))
<!-- 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="app"></div>
<!-- end snippet -->
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>
<!-- 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://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax
[2]: https://reactjs.org/docs/jsx-in-depth.html#spread-attributes
