Component State

What does setState do?

setState() schedules an update to a component’s state object. When state changes, the component responds by re-rendering.

What is the difference between state and props?

props (short for “properties”) and state are both just JavaScript objects that trigger a re-render when changed. While both hold information that influences the output of render, they are different in one important way: props get passed to the component (similar to function parameters) whereas state is managed within the component (similar to variables declared within a function).

Here are some good resources for further reading on when to use props vs state:

• Props vs State
• ReactJS: Props vs. State

Why is setState is giving me the wrong value?

Calls to setState are asynchronous - don’t rely on this.state to reflect the new value immediately after calling setState. Pass an updater function instead of an object if you need compute values based on the current state (see below for details).

Example of code that will not behave as expected:

incrementCount() {
  // Note: this will *not* work as intended.
  this.setState({count: this.state.count + 1});
}

handleSomething() {
  // this.state.count is 1, then we do this:
  this.incrementCount();
  this.incrementCount(); // state wasn't updated yet, so this sets 2 not 3
}

See below for how to fix this problem.

How do I update state with values that depend on the current state?

Pass a function instead of an object to setState to ensure the call always uses the most updated version of state (see below).

What is the difference between passing an object or a function in setState?

Passing an update function allows you to access the current state value inside the updater. Since setState calls are batched, this lets you chain updates and ensure they build on top of each other instead of conflicting:

incrementCount() {
  this.setState((prevState) => {
    return {count: prevState.count + 1}
  });
}

handleSomething() {
  // this.state.count is 1, then we do this:
  this.incrementCount();
  this.incrementCount(); // count is now 3
}

Learn more about setState

Should I use a state management library like Redux or MobX?

Maybe.

It’s a good idea to get to know React first, before adding in additional libraries. You can build quite complex applications using only React.