How to add multiple items to array with reactjs

Question

I have two classes. One holds the array, the other holds the array props. These are my classes:

//PARENT CLASS:

constructor() {
   super()

   this.state = {
      items: []
   }

   this.addItem = this.addItem.bind(this)
}

componentDidMount(){
   this.setState({
      items: [{
         name: 'Sebastian',
         num: '001'
      },{
         name: 'Josh',
         num: '002'
      }]
   })
}

addItem() {
??????
}

render() {
   return(
      <div>
        <MethodA items={this.state.items} addItem={this.addItem}/>
      </div>
   )
}

//CHILD CLASS:

function MethodA(props) {
   return(
      <div>
         {props.items.map((item, i) =>{
            return(<div key={i}>
               <span>{item.name}</span>
               <span>{item.num}</span>
            </div>)
         })}

         <button onClick={() => { props.addItem() }}>ADD ITEM</button>
      </div>
   )
}

Current result is like this:

<div>
   <span>Sebastian</span>
   <span>001</span>
</div>
<div>
   <span>Sebastian</span>
   <span>002</span>
</div>

Then after the "ADD ITEM" button was hit, this will be the new result:

<div>
   <span>Sebastian</span>
   <span>001</span>
</div>
<div>
   <span>Sebastian</span>
   <span>002</span>
</div>
<div>
   <span>New Name</span>
   <span>New Num</span>
</div>

I'm not sure whether what and how to use between push() or concat() or both. Any ideas?

Answer 1

Firstly, there's no need to set the initial state in componentDidMount, you can do it directly in constructor.

constructor(props) {
    super(props);

    this.state = {
      items: [
        {
          name: "Sebastian",
          num: "001"
        },
        {
          name: "Josh",
          num: "002"
        }
      ]
    };

    this.addItem = this.addItem.bind(this);
  }

To add an item you can use functional form of setState and you'll need to pass that item into callback from the child component.

addItem(item) {
    this.setState(state => ({
      items: [...state.items, item]
    }));
  }

// Child class
function MethodA(props) {
   return(
      <div>
         {props.items.map((item, i) =>{
            return(<div key={i}>
               <span>{item.name}</span>
               <span>{item.num}</span>
            </div>)
         })}

         <button onClick={() => props.addItem(item)}>ADD ITEM</button> // Pass item to the parent's method
      </div>
   )
}

Answer 2

Here's the deal. The difference between push() and concat() is in immutability.

If you use push on an array, it will mutate the original array and add a new value to that array (wrong).

If you use concat, it will create a new array for you, leaving the old array untouched (correct).

So you might want to do something along these lines:

addItem(item)
  this.setState(state => {
    const items = state.items.concat(item);
    return {
      items,
    };
  });
}