How is JavaScript used in Salesforce CPQ for custom pricing logic?

CPQ supports two JavaScript-based extension points for customisation:

1. Custom Price Methods (SBQQ.PricingPlugin): An Apex class implementing the SBQQ.PricingPlugin interface can override or supplement the standard calculator. The plugin receives the quote model as JSON, applies custom logic in Apex, and returns the modified model. While this is Apex (server-side), the plugin is invoked by the JavaScript-based CPQ calculator in the browser and can call back to Apex via the CPQ API.
2. Summary Variables can be used in Price Actions for aggregate calculations.
3. For UI-side customisation, CPQ supports Custom Script records (SBQQ__CustomScript__c) which allow JavaScript to run within the CPQ quote configurator page. These scripts can modify option visibility, field values, and UI behaviour in real-time — for example, displaying a custom warning based on a combination of selected options or auto-populating fields based on external data. Custom scripts require careful testing as they run in the browser's scope and can affect configurator performance.