The GraphQL Admin API lets you build apps and other integrations for the Shopify admin using GraphQL. With the API, you can create apps that offer functionality at every stage of a store’s operation, including shipping, fulfillment and product management.
Few Quick Starting Points:
An endpoint of GraphQL
https://my-store-name.myshopify.com/admin/api/graphql
How to get Access Token?
Login to your Shopify store and create a Private app in apps section and allow this new app to access your storefront data using the Storefront API.
How to test API GraphQL endpoint?
Test GraphQL by using the GraphiQL app and be sure that it works well for every aspect.
How to use JavaScript with Storefront API GraphQL?
There are many methods by which you can use GraphQL in shopify, and from those one is using Javascript. You can create a checkout URL by using JavaScript.