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Update the Players State Based on Previous State

3:01 with Guil Hernandez

In this video, we will update to a new state based on the previous state. That way we can be sure that this.state always holds the correct updated state.

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Update the players state using the concat() method:

handleAddPlayer = (name) => {
  let newPlayer = {
    name,
    score: 0,
    id: this.prevPlayerId += 1
  };
  this.setState( prevState => ({
    players: prevState.players.concat(newPlayer)
  }));
}

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If the handleAddPlayer function we wrote in the previous video still feels a bit 0:00

confusing, don't worry, it was confusing for me too at first. 0:04

So let's take a little time here to unpack this function, 0:07

to help drive home the logic. 0:10

Now, anytime I'm examining how a piece of code works, I like to use console.log and 0:12

remove or change parts of the code to see how it affects the program or the UI. 0:17

So if I log the spread operator that's bringing in the players 0:22

state to the console with ...this.state.players, 0:28

then submit a new player, notice how the console logs all but 0:33

the new player being added to state. 0:38

That's because the spread operator, followed by this.state.players, 0:41

represents the players in state before state was updated. 0:46

To better demonstrate, 0:50

I'll comment out the spread operator from the players array. 0:52

Over in the scoreboard, when I submit a new player, notice how the original 0:56

players disappear from the scoreboard, and we're left with just the new player. 1:01

They've also been removed from state. 1:07

And if you include the new player object first in the array and 1:12

the spread operator below the object, 1:17

notice how each new player gets added to the top of the list. 1:20

Include the spread as the first item of the array, and 1:30

new players get added to the bottom of the list. 1:34

Again, the spread operator here represents all the existing 1:42

players on the scoreboard before a new player is submitted. 1:46

With that in mind, you've learned that it's usually a good idea to update to 1:51

a new state based on the previous state. 1:55

That way we can be sure that this.state always holds the correct updated state. 1:57

So let's update setState to take a callback 2:03

function with the parameter prevState. 2:07

We'll wrap the players a rate in a return statement with curly braces. 2:11

Then replace this.state.players with prevState.players. 2:20

Again, the setState callback function is React's recommended way of updating state. 2:27

In a small and simple scoreboard app like this, 2:34

you likely won't run into state inconsistency issues. 2:37

But if at some point you have 10's or 100 player objects in state, this method 2:40

provides a better way to verify that the state did indeed update correctly. 2:45

In the teacher's notes, I have posted an example of yet 2:53

another way you might update the player state using the JavaScript concat method 2:56

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