There's a pattern I've seen in almost every Express codebase I've worked in. It starts small.

You need to log a correlation ID inside a service function. So you add req as a parameter. Then the function that calls it needs req . Then the one above that. Within a few weeks, half your codebase takes req as a parameter even though it only ever reads one field from it.

// This is where it starts async function processOrder ( orderId : string , req : Request ) { logger . info ( ' Processing order ' , { correlationId : req . headers [ ' x-correlation-id ' ] }); await notifyWarehouse ( orderId , req ); // now warehouse needs req too await sendEmail ( orderId , req ); // and email await auditLog ( ' order.processed ' , req ); // and audit }

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Every function signature is polluted. Tests need a mock req . Services are coupled to Express internals.…