How to update an item in array while setting state in react

For a better practice first of all you should avoid mutating state and if you are gonna need values from state while updating state you should consider using functional state updates. This will help on getting the correct values always.

Another thing to consider is that you are using this.state.score right after setting it. setState is async and can occur after you do your if statement. For this you should consider using callbacks.

Below is the modified version of your code with the suggestions above;

this.setState((prevState) => {
  const mushrooms = Object.assign({}, prevState.mushrooms);
  mushrooms[foundMushroom.key].remaining = false;
  return { mushrooms, score: (prevState.score + 1) };
}, () => {
  if (this.totalMushrooms === this.state.score) {
    this.props.setTotalTime(startTime, steps);
    this.props.history.push("/score");
  }
});

I don't know how are you using this.state.mushrooms value but for better performance you can do a little change. If you want to modify only one property then you should move your properties one level up. mushrooms property is unnecessary in my opinion.

Example Data

Rather then using like below

this.state = {
  mushrooms: {
    mushA: {
      remaining: true
    },
    mushB: {
      remaining: false
    },
    mushC: {
      remaining: true
    }
  }
};

You can use like this

this.state = {
  mushA: {
    remaining: true
  },
  mushB: {
    remaining: false
  },
  mushC: {
    remaining: true
  }
};

This way you can update your state like below. One property at a time and I believe this would result in a better performance updates.

this.setState((prevState) => {
  const mushroom = Object.assign({}, prevState[foundMushroom.key], { remaining: false });
  return { [foundMushroom.key]: mushroom, score: (prevState.score + 1) };
}, () => {
  if (this.totalMushrooms === this.state.score) {
    this.props.setTotalTime(startTime, steps);
    this.props.history.push("/score");
  }
});