What do these three dots in React do?


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).

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 properties are in props are included.

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" :