How to throw error using nest js guards without using any authentication module..?

Wanna send custom error in nestjs guards.

import { CanActivate, Injectable, ExecutionContext, NotFoundException } from '@nestjs/common';
import { Observable } from 'rxjs';
import { InjectModel } from '@nestjs/mongoose';
import { Model } from 'mongoose';
import { UserParamsNotFoundException } from 'src/statusResponse/error.response';


@Injectable()
export class UserGuard implements CanActivate {
    constructor(
        @InjectModel(Users.name) private userModel: Model<CreateUser>,
    ) {}
    async canActivate(
        context: ExecutionContext,
    ): Promise<any> {
        const request = context.switchToHttp().getRequest();

        const { user, } = request.body; // u can extract the key using object destructing .
        const isUserExist: boolean = function (); // which will return true or false;
        
        return isUserExist ? true : false;


    }
};

Recently i am working on a project where there's no need of authentication. But, i have to check whether user exists in db or not before doing any CRUD operations. I used guard as decorator to tackle the problem. Pls find the below solution.

import { CanActivate, Injectable, ExecutionContext, NotFoundException } from '@nestjs/common';
import { Observable } from 'rxjs';
import { InjectModel } from '@nestjs/mongoose';
import { Model } from 'mongoose';
import { UserParamsNotFoundException } from 'src/statusResponse/error.response';


@Injectable()
export class UserGuard implements CanActivate {
    constructor(
        @InjectModel(Users.name) private userModel: Model<CreateUser>,
    ) {}
    async canActivate(
        context: ExecutionContext,
    ): Promise<any> {
        const request = context.switchToHttp().getRequest();

        const { user, } = request.body; // u can extract the key using object destructing .
        const isUserExist: boolean = function (); // which will return true or false;

        if (!isUserExist) throw new NotFoundException('Oops User not exist. Try again');
        else return true;
    }
};

Hope this is helpful..!!!