ContextUtils

Kind: Class

Source: packages/core/helpers/context-utils.ts

Part of: Core

ContextUtils centralizes reflection and execution-context helpers used when NestJS resolves handler parameters. It reads callback metadata, aligns parameter metadata with reflected types, builds argument placeholders, and creates ExecutionContextHost instances for custom parameter factories.

Methods

Where it refuses work

• ContextUtils stops the work with an early return when !paramtypes.

Diagram

mermaid
graph LR
  Handler[Controller/Resolver Handler] --> Reflection[Reflect Metadata]
  Reflection --> ContextUtils
  ContextUtils --> ParamTypes[Parameter Metatypes]
  ContextUtils --> Arguments[Argument Array]
  ContextUtils --> ContextFactory[ExecutionContextHost Factory]
  ContextFactory --> CustomFactory[Custom Parameter Factory]
  Arguments --> Handler
  CustomFactory --> Handler

Usage

ts
import { ContextUtils } from '@nestjs/core/helpers/context-utils';
import { ExecutionContextHost } from '@nestjs/core/helpers/execution-context-host';

class UsersController {
  findOne(id: string) {
    return { id };
  }
}

const contextUtils = new ContextUtils();
const controller = new UsersController();

// Read TypeScript design:paramtypes metadata for the handler.
const paramTypes = contextUtils.reflectCallbackParamtypes(
  controller,
  'findOne',
);

// Create an execution-context factory for custom parameter decorators.
const createContext = contextUtils.getContextFactory('http');

const args = [{ params: { id: '42' } }, {}, () => undefined];
const context: ExecutionContextHost = createContext(args);

console.log(paramTypes); // Example: [String]
console.log(context.getType()); // "http"

AI Coding Instructions

• Use reflectCallbackParamtypes() and reflectCallbackMetadata() instead of reading Reflect metadata directly in parameter-resolution code.
• Keep parameter indexes aligned when using getArgumentsLength(), createNullArray(), and mergeParamsMetatypes().
• Create contexts through getContextFactory() so the resulting ExecutionContextHost has the correct transport type.
• Wrap custom parameter decorators with getCustomFactory() to ensure they receive both decorator data and the generated execution context.
• Treat missing reflection metadata as valid; handlers may not have emitted design-time metadata.

Used by

3 references from 3 files. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

Imported by (3)

• RpcHandlerMetadata — packages/microservices/context/rpc-context-creator.ts:35
• WsHandlerMetadata — packages/websockets/context/ws-context-creator.ts:34
• MessageMappingProperties — packages/websockets/gateway-metadata-explorer.ts:15