Inferring Types

It is often useful to wrap functionality of your @trpc/client or @trpc/react api within other functions. For this purpose, it’s necessary to be able to infer input types, output types, and api paths generated by your @trpc/server router.

Inference Helpers

@trpc/server exports the following helper types to assist with inferring these types from the AppRouter exported by your @trpc/server router:

  • inferProcedureOutput<TProcedure>
  • inferProcedureInput<TProcedure>
  • inferSubscriptionOutput<TRouter, TPath>

Example Code

utils/trpc.ts

  1. // trpc-helper.ts
  2. import type { AppRouter } from 'api/src/routers/_app' // Import AppRouter from your main server router
  3. import type { inferProcedureOutput, inferProcedureInput, inferSubscriptionOutput } from '@trpc/server'
  5. /**
  6.  * Enum containing all api query paths
  7.  */
  8. export type TQuery = keyof AppRouter['_def']['queries']
  10. /**
  11.  * Enum containing all api mutation paths
  12.  */
  13. export type TMutation = keyof AppRouter['_def']['mutations']
  15. /**
  16.  * Enum containing all api subscription paths
  17.  */
  18. export type TSubscription = keyof AppRouter['_def']['subscriptions']
  20. /**
  21.  * This is a helper method to infer the output of a query resolver
  22.  * @example type HelloOutput = InferQueryOutput<'hello'>
  23.  */
  24. export type InferQueryOutput<TRouteKey extends TQuery> = inferProcedureOutput<
  25.   AppRouter['_def']['queries'][TRouteKey]
  26. >
  28. /**
  29.  * This is a helper method to infer the input of a query resolver
  30.  * @example type HelloInput = InferQueryInput<'hello'>
  31.  */
  32. export type InferQueryInput<TRouteKey extends TQuery> = inferProcedureInput<
  33.   AppRouter['_def']['queries'][TRouteKey]
  34. >
  36. /**
  37.  * This is a helper method to infer the output of a mutation resolver
  38.  * @example type HelloOutput = InferMutationOutput<'hello'>
  39.  */
  40. export type InferMutationOutput<TRouteKey extends TMutation> =
  41.   inferProcedureOutput<AppRouter['_def']['mutations'][TRouteKey]>
  43. /**
  44.  * This is a helper method to infer the input of a mutation resolver
  45.  * @example type HelloInput = InferMutationInput<'hello'>
  46.  */
  47. export type InferMutationInput<TRouteKey extends TMutation> =
  48.   inferProcedureInput<AppRouter['_def']['mutations'][TRouteKey]>
  50. /**
  51.  * This is a helper method to infer the output of a subscription resolver
  52.  * @example type HelloOutput = InferSubscriptionOutput<'hello'>
  53.  */
  54. export type InferSubscriptionOutput<TRouteKey extends TSubscription> =
  55.   inferProcedureOutput<AppRouter['_def']['subscriptions'][TRouteKey]>
  57. /**
  58.  * This is a helper method to infer the asynchronous output of a subscription resolver
  59.  * @example type HelloAsyncOutput = InferAsyncSubscriptionOutput<'hello'>
  60.  */
  61. export type InferAsyncSubscriptionOutput<TRouteKey extends TSubscription> =
  62.   inferSubscriptionOutput<AppRouter, TRouteKey>
  64. /**
  65.  * This is a helper method to infer the input of a subscription resolver
  66.  * @example type HelloInput = InferSubscriptionInput<'hello'>
  67.  */
  68. export type InferSubscriptionInput<TRouteKey extends TSubscription> =
  69.   inferProcedureInput<AppRouter['_def']['subscriptions'][TRouteKey]>