Function: defineMiddleware()

function defineMiddleware<TContextIn, TDerived>(derive): MiddlewareDefinition<TContextIn, TDerived>;

Defined in: packages/core/src/core/middleware/ContextDerivation.ts:123

Define a context-deriving middleware.

The returned object can be used in two ways:

1. As a type-level constraint (the generics carry the derived context)
2. As a runtime middleware via .toMiddlewareFn() (for the existing pipeline)

Type Parameters

Type Parameter	Description
TContextIn	The input context type
TDerived extends Record<string, unknown>	Auto-inferred derived properties

Parameters

Parameter	Type	Description
derive	(ctx) => TDerived | Promise<TDerived>	Function that inspects context and returns derived data

Returns

MiddlewareDefinition<TContextIn, TDerived>

A MiddlewareDefinition with the derived type encoded

Example

// Auth middleware that derives `user`
const requireAuth = defineMiddleware(async (ctx: { token: string }) => {
    const user = await db.getUser(ctx.token);
    if (!user) throw new Error('Unauthorized');
    return { user };
});

// Rate limit middleware that derives `rateLimitInfo`
const rateLimit = defineMiddleware(async (ctx: { ip: string }) => {
    const info = await checkRateLimit(ctx.ip);
    if (info.exceeded) throw new Error('Rate limited');
    return { rateLimitInfo: info };
});

See

MiddlewareFn for the standard middleware type