Interceptor

Interceptors have a set of useful capabilities which are inspired by the Aspect Oriented Programming (AOP) technique.

Usage

They make it possible to:

• bind extra logic before / after method execution
• transform the result returned from a function
• transform the exception thrown from a function
• extend the basic function behavior
• completely override a function depending on specific conditions (e.g., for caching purposes)

Aspect interception

• Use an interceptor to log user interaction (e.g., storing user calls, asynchronously dispatching events or calculating a timestamp).

    import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
    import { Observable } from 'rxjs';
    import { tap } from 'rxjs/operators';

    @Injectable()
    export class LoggingInterceptor implements NestInterceptor {

        intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
            console.log('Before...');

            const now = Date.now();
            return next
            .handle()
            .pipe(
                tap(() => console.log(`After... ${Date.now() - now}ms`)),
            );
        }
    }