Quantity is connected together not summed up

Question

I have a code where when the add button is clicked, it sums up the value of two objects properties and update it. When you use a "+" button to add an item, it works but whenever the user manually inputs the amount to add, instead of adding them up, it connects it like a string.

Example:

Where 5100 + 122222 should be 127322. Here's my code snippet:

JS Function:

var tobeSaved = store.getters.printCart;

    if(tobeSaved.length){
      if(sessionStorage.length){
        var existingCart = JSON.parse(sessionStorage.cart);
        var size = Object.keys(existingCart).length;
        //console.log(existingCart);
        tobeSaved.forEach(tobeSaved => {
          let obj = existingCart.find(ob => ob.itemID === tobeSaved.itemID);
          console.log(obj);
          if(obj!==undefined){
            obj.itemQuantity+=tobeSaved.itemQuantity;
            existingCart.itemQuantity = obj.itemQuantity;
            let myObj_serialized = JSON.stringify(existingCart);
            sessionStorage.setItem('cart', myObj_serialized);
          } else{
            existingCart.push(tobeSaved);
            let myObj_serialized = JSON.stringify(existingCart);
            sessionStorage.setItem('cart', myObj_serialized);
          }
      })
      
      } 
      else{
        let myObj_serialized = JSON.stringify(tobeSaved);
        sessionStorage.setItem('cart', myObj_serialized);
      }
    }
  }

EDIT: Here's the User Input part

<div class="product__details__quantity d-flex justify-content-center">
   <div class="quantity">
       <div class="pro-qty">
           <input type="number" v-model="quantity" >
       </div>
   </div>
</div>

Edit #2: Added vuex code

const store = new Vuex.Store({
state:{
  cartItem: [],
  cartCount: 0
},
mutations:{
    addCart(state, payload){
        this.state.cartItem.push(payload);
    },
    addCount(state, payload){
      state.cartCount+=payload
    },
},
actions:{

},
getters:{
    printCart(state){
        return state.cartItem;
    },
    counter(state){
      return state.cartCount;
    }
}

});

EDIT #3. Here's the button that passes the data to vue

<button type="button" class="btn btn-success btn-block" data-dismiss="modal" @click="addCart({
   itemID: selectedUser.id,
   itemName: selectedUser.item_name,
   itemPrice: variant.default_price,
   itemQuantity: quantity
 })">Add to Cart</button>

AddCart:

addCart: function(cartdets){
    this.quantity = 1;
    var cartItems = store.getters.printCart;
    //console.log(cartItems);
    var filtercart = cartItems.some(function(cart){
      return cart.itemID === cartdets.itemID
    });
    //console.log(filtercart);
      if(!filtercart){
        store.commit('addCart', cartdets);
        store.commit('addCount', 1);
      } else{
        store.commit('addCart', cartdets);
      }
  }