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?

I can see you're not passing name & num as the parameters to the add item method. Once you pass them, then inside addItem() you can simply mutate the items array in state. But keep in mind that items is array and arrays are reference types in JS. So you first need to make a copy of items array before actually concatenating the new item in the existing array. — Mohit
– Mohit, Commented Oct 19, 2019 at 7:54

Clarity · Accepted Answer · 2019-10-20 10:24:56Z

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>
   )
}

Andrej Jurkin · Accepted Answer · 2019-10-19 08:18:02Z

0

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,
    };
  });
}

answered Oct 19, 2019 at 8:18

Andrej Jurkin

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