Magento 2 module to sync from the GraphQL session to the Knockout session and from the Knockout session to the GraphQL session.

• The GraphQL cart token (aka masked_id) is added to the CustomerDatas section cart of the Knockout frontend (customerData.get('cart')) which is automatically synced by Knockout to local storage;
• Within local storage, you can fetch the GraphQL token again from mage-cache-storage.cart.masked_id, for instance if you are switching from a regular Knockout frontend to a React-based checkout;

Thanks to Willem Wigman for coming up with the idea for putting the cart token in the section data.

Use a GraphQL client within React to generate a cart token:

mutation {
  createEmptyCart
}

The result might be something like the following:

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You should now be able to navigate to the Knockout frontend. After making a modification to the cart (adding a new item, changing the quantity or just wiping out local storage), you can then inspect the local storage entry mage-cache-storage.cart.masked_id: It should hold the same token as mentioned above.

If you add a product to the cart in the Knockout frontend, the same product should be there in the GraphQL session as well:

query cart($cartId: String!) {
  cart(cart_id: $cartId){
    id
    items {
      product {
        sku
      }
    }
  }
}

Now, add a product to the cart (in this case with a product SKU 24-MB04:

mutation addToCart($cartId: String!) {
  addSimpleProductsToCart(input: {
    cart_id: $cartId, 
    cart_items: [{data: {quantity: 1, sku: "24-MB04"}}]
  }) {
    cart {
      id
      total_quantity
      items {
        id
        prices {
          row_total {
            currency
            value
          }
        }
        product {
          url_key
        }
      }
    }
  }
}

Next, head over to the Knockout frontend, wipe out local storage mage-cache-storage.cart (or do this in Knockout via customerData.clear()) and inspect the cart again.

Use a GraphQL client within React to generate a customer token:

mutation login($email:String!, $password:String!) {
  generateCustomerToken(email:$email, password:$password) {
    token
  }
}

Next, login as the same customer in the Knockout frontend. The local storage item mage-cache-storage.customer.customer_token now refers to the same GraphQL token.

This probably needs some enhancement, so that you are logged in right away in either frontend, without loosing security.