In React, the state is a source of data that drives the UI of a component. The state is managed by the component itself, and it is used to store and update the data that is displayed in the component’s UI.
Updating the state and re-rendering the UI is an important part of building applications with React, and it is done using the setState()
method. The setState()
method is a built-in method of a component that allows you to update the state and re-render the component’s UI.
In this article, we will explore the setState()
method in more detail and see how it is used to update the state and re-render elements in React.
Updating the State
To update the state of a component, you need to use the setState()
method. The setState()
method takes an object as an argument and merges it with the current state of the component.
For example, consider the following component that has a state with a count
property:
class Counter extends React.Component {
constructor(props) {
super(props);
this.state = {
count: 0
};
}
render() {
return (
<div>
<h1>{this.state.count}</h1>
<button onClick={() => this.setState({ count: this.state.count + 1 })}>
Increment
</button>
</div>
);
}
}
In this example, the Counter
component has a state with a count
property that is initialized to 0
. The component also has a button
element with an onClick
prop that calls the setState()
method when it is clicked.
When the setState()
method is called, it updates the count
property of the state and re-renders the component’s UI. In this case, the component’s UI is the h1
element that displays the count
value, and it will be updated every time the button is clicked.
Re-Rendering Elements
When the state of a component is updated, the component’s UI is re-rendered to reflect the changes in the state. The component’s render()
method is called again, and it returns a new tree of elements based on the updated state.
For example, consider the following component that has a state with a text
property:
class TextBox extends React.Component {
constructor(props) {
super(props);
this.state = {
text: ''
};
}
render() {
return (
<div>
<input value={this.state.text} onChange={(e) => this.setState({ text: e.target.value })} />
<p>{this.state.text}</p>
</div>
);
}
}
In this example, the TextBox
component has a state with a text
property that is initialized to an empty string. The component also has an input
element with a `value prop and an onChange
prop, and a p
element that displays the text
value.
When the user types in the input
element, the onChange
prop is called and the setState()
method is called with the updated text
value. This updates the text
property of the state and re-renders the component’s UI.
In this case, the component’s UI is the input
element and the p
element, and they will be updated every time the user types in the input
element.
Asynchronous Updating
The setState()
method is asynchronous, which means that it may not immediately update the state of the component. This is because the setState()
method is designed to be efficient and avoid unnecessary re-renders.
To update the state in a synchronous and predictable way, you can use the setState()
method with a callback function or with the prevState
parameter.
For example, consider the following code that updates the count
property of the state in a synchronous and predictable way:
this.setState((prevState) => ({ count:
prevState.count + 1 }));
In this example, the setState()
method is called with a callback function that takes the prevState
as an argument and returns the updated state object. The callback function is called after the state has been updated, and it ensures that the count
property is updated in a synchronous and predictable way.
Conclusion
By understanding the asynchronous nature of the setState()
method and using it with a callback function or the prevState
parameter, you can update the state and re-render the UI in a reliable and efficient way.
Exercises
To review these concepts, we will go through a series of exercises designed to test your understanding and apply what you have learned.
What is the state in React?
The state in React is a source of data that drives the UI of a component. The state is managed by the component itself, and it is used to store and update the data that is displayed in the component’s UI.
How do you update the state of a component in React?
To update the state of a component in React, you need to use the setState(
) method. The setState()
method takes an object as an argument and merges it with the current state of the component.
How do you re-render the UI of a component in React?
To re-render the UI of a component in React, you need to update the state of the component using the setState()
method. When the state is updated, the component’s render()
method is called again, and it returns a new tree of elements based on the updated state.
Is the setState()
method synchronous or asynchronous?
The setState()
method is asynchronous, which means that it may not immediately update the state of the component. This is because the setState()
method is designed to be efficient and avoid unnecessary re-renders.
How do you update the state in a synchronous and predictable way in React?
To update the state in a synchronous and predictable way in React, you can use the setState()
method with a callback function or with the prevState
parameter. For example, you can use the setState()
method with a callback function that takes the prevState
as an argument and returns the updated state object. This ensures that the state is updated in a synchronous and predictable way.