Startup.cs

using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.SpaServices.ReactDevelopmentServer;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.IdentityModel.Tokens;
using pawsitive.Data;
using pawsitive.EntityModels;
using System.Text;

namespace pawsitive
{
    public class Startup
    {
        public Startup(IConfiguration configuration, IWebHostEnvironment env)
        {
            Configuration = configuration;
            _env = env;
        }

        public IConfiguration Configuration { get; }
        private readonly IWebHostEnvironment _env;

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {

            services.AddControllersWithViews();
            services.AddSwaggerGen();

            services.AddControllersWithViews()
                .AddNewtonsoftJson(options =>
                options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
            );

            // For Entity Framework
            if (_env.IsDevelopment())
            {
                services.AddDbContext<DataContext>(options => options.UseSqlServer(Configuration.GetConnectionString("LocalDatabase")));
            }
            else
            {
                services.AddDbContext<DataContext>(options => options.UseSqlServer(Configuration.GetConnectionString("ProductionDatabase")));
            }

            services.AddTransient<DataManager>();

            // For Identity  
            services.AddIdentity<User, IdentityRole>()
                .AddEntityFrameworkStores<DataContext>()
                .AddDefaultTokenProviders();

            // Adding Authentication  
            services.AddAuthentication(options =>
            {
                options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
                options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
                options.DefaultScheme = JwtBearerDefaults.AuthenticationScheme;
            })

            // Adding Jwt Bearer  
            .AddJwtBearer(options =>
            {
                options.SaveToken = true;
                options.RequireHttpsMetadata = false;
                options.TokenValidationParameters = new TokenValidationParameters()
                {
                    ValidateIssuer = true,
                    ValidateAudience = true,
                    ValidAudience = Configuration["JWT:ValidAudience"],
                    ValidIssuer = Configuration["JWT:ValidIssuer"],
                    IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(Configuration["JWT:Secret"]))
                };
            });

            // In production, the React files will be served from this directory
            services.AddSpaStaticFiles(configuration =>
            {
                configuration.RootPath = "ClientApp/build";
            });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            app.UseSwagger();

            app.UseSwaggerUI(c =>
            {
                c.SwaggerEndpoint("/swagger/v1/swagger.json", "Pawsitive API V1");
            });

            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Error");
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();
            app.UseSpaStaticFiles();

            app.UseRouting();

            app.UseAuthentication();
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller}/{action=Index}/{id?}");
            });

            app.UseSpa(spa =>
            {
                spa.Options.SourcePath = "ClientApp";

                if (env.IsDevelopment())
                {
                    // remove this to run client side separately
                    //spa.UseReactDevelopmentServer(npmScript: "start"); 

                    spa.UseProxyToSpaDevelopmentServer("http://localhost:3000");
                }
            });
        }
    }
}