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JavaScript React by Example Building the Application Adding Guests to the List

Carey Cade
Carey Cade
5,283 Points

Why not use array push() method?

Why don't we just use an array push() method to add the new guest object into the state.guests array?

Thanks:)

5 Answers

Seth Kroger
Seth Kroger
56,392 Points

Thou Shalt Not Alter State Directly.

That's pretty much a commandment-level dictum in React. Every time you want to change it you should use setState() handing it the new info to put there. Since push() adds to the existing array instead of returning a new one it goes against that principle.

https://facebook.github.io/react/docs/state-and-lifecycle.html#using-state-correctly

Carey Cade
Carey Cade
5,283 Points

Hey Seth! I know that of the importance of using setState(). In theory, couldn't one add to the existing array, and then setState() to be that new array? Is that incorrect... or just not best practice? Thanks so much!

Seth Kroger
Seth Kroger
56,392 Points

push() will modify the current array in state. There won't be a "new" array, just a changed old one and that would be incorrect with the way React wants to do things.

Anthony Albertorio
Anthony Albertorio
22,169 Points

Montalvo Miguelo You are still modifying state directly, via modifying the pervious state. From what I understand, modifying any sort of state directly is not recommended in React.

See the React docs for more information: https://reactjs.org/docs/state-and-lifecycle.html

Also, see this great post: https://medium.com/@shopsifter/using-a-function-in-setstate-instead-of-an-object-1f5cfd6e55d1

A better way could be by just updating the new state like this:

class App extends Component {
//...

  newGuestSubmitHandler = e => {
    e.preventDefault();
    if (this.state.pendingGuest !== "") {
      const newPerson = {
        name: this.state.pendingGuest,
        isConfirmed: false,
        isEditing: false
      };
      this.setState(prevState => ({
        guests: [newPerson, ...prevState.guests],
        pendingGuest: ""
      }));
    }
  }

//...
}
Montalvo Miguelo
Montalvo Miguelo
24,789 Points

You can pass a function to setState method, that function receives the prevState and props as arguments so you could use push as follows:

class App extends Component {
  //...
  newsGuestSubmitHandler(event) {
    event.preventDefault();

    this.setState((prevState, props) => {
      prevState.guests.push({
        name: this.state.pendingGuest,
        isEditing: false,
        isConfirmed: false
      });

      return {
        guests: prevState.guests,
        pendingGuest: ''
      }
    });
  }
}
Charles Wanjohi
Charles Wanjohi
9,235 Points

Array push method would add the new guest object at the end of the array while the intention here is to add it as the first object (index 0) in the array.Otherwise its still possible but the new object would be added at the end i.e it wont be rendered as the first guest component.The implementation would be a little different though

Carey Cade
Carey Cade
5,283 Points

Thanks Charles!

Aaron Smith
Aaron Smith
19,091 Points

I initially wrote my new guest handler like Carey was thinking, using unshift instead of push so that the new guest object gets added to the first position in the array:

  addNewGuest = e => {
    e.preventDefault();
    this.state.guests.unshift({
      name: this.state.pendingGuest,
      isConfirmed: false,
      isEditing: false
    });
    this.setState({
      pendingGuest: ''
    });
  }

I tested it and got the same functionality Guil got in the video using the spread operator inside of the setState() function. I can accept that this is not best practice, but I'm wondering if there is any actual disadvantage to coding it this way.