React, conditional setState with the click of a button

So, I don't know if how I want this to work is possible at all, but in my head I think it could I just can't seem to get it to work..

Basically I have an empty array of objects:

this.state = {
     orders: [{}]
}

Then when I click on a button I want an order to be added (custom event because cart has no parent-child relations to the button clicked, for reasons):

pubSub.on("addToCart", (data) =>
   this.setState((prevState => ({
      orders: [...prevState.orders, data.order]
   })
);

This works. I end up with:

orders: [
 {[sku]: sku, quantity: quantity}
]

Now the problem I am stuck with is that if an order is to be added with an SKU that is already in the array, I want just the quantity to be updated. If the SKU isn't already found in the array, I want a new order added. So I want to check the array for the existence of a key (key SKU is the same as the value of key SKU).

So, to check an array for the existence of a key I would use:

this.state.orders.map(obj =>{
  if (!(data.sku in obj)){
    **setState here with order added**
  }
})

I know how the conditional should look like if it finds that the key exists, and then only update the quantity.

the problem I face is that it iterates over all the orders in the array, and every time the key doesn't exist, it adds a new order. How could I make this, so it checks for the existence of a key, and only adds a new order 1 time if that's the case, and not every time? I want actually to first check all the keys, and then decide what to do. With this approach, I have to decide every round of the map what should be done.

Also keep in mind that we are in the scope of a custom event handler, where I want to use data. inside this handler, I cannot assign variables outside the scope of the map function. So basically it's or I use just setState, or I use the map function and inside there the setState. I can't use them both next to each other as data is only available in the first method call. This is also the first time I try to use this custom event handler thing, and the scoping has tied my hands a bit.

If this makes sense, and someone can point me in the right direction, that would be greatly appreciated. Thanks in advance!

You can pass function in setState. This function will have current state and, if you use classes, current props. In this function you create state change - part of state object that you want to update. In your case it might look like this (it's barebones and dirty, to show how you can do it):

// data is here, somwhere
this.setState(function(state, props) {
  let index = state.orders.findIndex(obj => data.sku in obj);
  if (index > -1) {
    // update existing
    return {orders: state.orders.slice(0, index).concat(
      {...state.orders[index], quantity: state.orders[index].quantitiy + 1},
      state.orders.slice(index+1))}
  } else {
    // create new
    return {orders: state.orders.concat(data)}
  }
})