Wire service syntax and reactive parameters in store components
Key Takeaway
To react to property changes, a wire configuration value must be a $-prefixed string. Until every reactive parameter has a value, the wire provisions neither data nor an error.
The wire service is the declarative way a Lightning web component reads Salesforce data. You import a wire adapter and decorate a property or function with @wire, and the framework delivers a stream of data to it. Reactive parameters, marked with a $ prefix in the configuration object, make the wire provision fresh data automatically whenever a component property changes. In a store component, you use the same syntax to read data such as the cart total, and you change data with imperative calls.
What the wire service does
The wire service is reactive and built on Lightning Data Service (LDS). Data reaches the component as an immutable stream, where each new value is a newer version of the one before. The wire service provisions data rather than fetches it because no network request may be needed if the data is already in the client cache.