Why my setState doesn’t work?

Confused man

Photo by Bruce Mars

setState is one of the most essential operations in React. Yet, it is one of the most confusing. If you are new to React, then you may feel that it does not always do what you want it to do. It is almost as it doesn’t work. The problem may go like this:

You set a state with a setState

   this.setState({count: 1})

Then you follow it with another setState which references the previous state value

   this.setState({count: this.state + 1})

Then you check the state value, and it is not what you expect it to be

   colsole.log(this.state);  // Prints 1

What? Is setState broken?

Keep calm. setState is fine. Keep reading, and I will help why that is happening and how to avoid that.

First of all, let’s have a look what React documentation says about setState:

React may batch multiple setState() calls into a single update for performance.

Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.

State updates in React are not applied immediately. Instead, they are placed in a queue and scheduled. In fact, React does not apply the new values to the state until the component is reconciled. Moreover, React Fiber architecture introduced in React 16 may rearrange the component update queue, putting more important updates (like animation) ahead of less important. All that means that you can’t rely on your new state values you apply in setState to be applied immediately.

Therefore, if you change the component’s state with setState, there’s a good chance that your changes won’t be applied immediately but instead will be scheduled to happen sometime in future.

// Before this this.state.count is 0
this.setState({count: 1});
// Read this.state values immediately after updating them
console.log(this.state.count); // May print 0 or 1

Rest assured, even in the worst case scenario your changes will be applied once the reconciliation process is completed. Because React is very efficient, the update does not take long to come through. It takes micro- or milliseconds to see your changes. But still, it is not immediate. In most cases that tiny delay does not matter.

However, if your setState depends on the previous state values, you may be in trouble.

// Before this this.state.count is 0
this.setState({count: 1});
this.setState({count: this.state.count + 1});
console.log(this.state.count);

Would you like to make a guess about what is printed in the last line? The problem is, you can never be sure. It may be 2 if the changes from the first setState have come through. Or it may be 1 if the first setState was scheduled for the later execution.

How do you deal with that?

Turns out, there’s a special syntax for that. One of React’s best-kept secrets is that setState comes in two flavours 1.

The first form takes an object as a parameter and updates the state according to its values. It does not return a value.

For example, this will update this.state.count to 1 and this.state.tempo to 120. If there are any other values in this.state, they will be left unchanged.

this.setState({count: 1, tempo: 120});

The second form, known as functional setState, is more complex. Instead of an object, it takes a function as a parameter.

function setStateFunction(state, props) {
  const newState = {...state, count: state.count + 1};
  return newState;
}
this.setState(setStateFunction);

The function receives two parameters: the current state and props. Contrary to the synchronous form, it has to return the state object which then becomes your new state.

Note the ...state spread operator in this line

const newState = {...state, count: state.count + 1};

Whatever the state function returns becomes your new state. Therefore it is very important that you return the complete state. It needs to contain not only the modified values but the other state values as well. Spread operator copies all values from your state into the new object so that you don’t lose any of your state values.

If your setState depends on the current value of the state or props, you need to use the functional (the second) form.

In other words, if you find yourself writing this.state or this.props inside your setState update, you need to use the functional form and obtain the current state and props values from state and props function parameters.

// Before this this.state.count is 0
this.setState({count: 1});

// Use the functional setState form
function incrementCount(state, props) {
  return {...state, count: state.count + 1};
}
this.setState(incrementCount);
console.log(this.state.count); // Always prints 2

React guarantees that the functional setState calls will be executed in the order they were called.

Therefore, the values of state and props arguments of the setState’s update functions will always be “correct”.

We can rewrite the previous example using an inline arrow function to make it a bit simpler:

// Before this this.state.count is 0
this.setState({count: 1});

// Use the functional setState form
this.setState(state => ({...state, count: state.count + 1}));
console.log(this.state.count); // Always prints 2

Let’s recap what we learned:

  1. setState calls are not guaranteed to be applied immediately.
  2. There are two forms of setState: one takes an object, and the other takes a function.
  3. If your setState relies on state or props values, you need to use the functional form.
  4. Never refer to this.state or this.props inside your setState. Instead use state and props arguments of the setState`s update function.

Happy coding!

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  1. Well, not really a secret. In fact, React team tells everyone who wants to listen about the setState’s duality. Still surprisingly few people know about that. [return]