Create your JSS app

 I am going to follow code first approach here

Step 1: Identify Framework

We are ready to create a app. . The first step is to identify the template that matches your framework (Angular, React, or Vue).

Step 2: Create a app

Ø  Open a command prompt.

Type the create app command for your framework/library in the form:
jss create <your-app-name> <app-template-name>

Ø  jss create demoapp react

Ø  Change to your app's directory. For example:
cd demoapp

Ø  Type the command to start the app in disconnected mode (which uses the defined content located on the local development server):
jss start

It automatically launches your app in the default browser, http://localhost:3000.

This is called Disconnected Mode.

Step 3: Connect a app with sitecore

• Run this command

Jss deploy config

• You can see the changes in your wwwroot folder

• Run this command

Jss deploy app –c –d

Ohh, gor error

• Run this

jss deploy component Form --allowedPlaceholders jss-main --acceptCertificate yourCertThumbPrint

• You can get your certificate here,
• Run this

jss deploy app --acceptCertificate FD:12:F0:2E:42:C5:48:F3:58:39:03:27:F7:CF:45:DC:AF:DB:72:91 -c –d

• Once it done, we can see the changes in our sitecore js url

Welcome to Sitecore JSS (dev.local)

This is called connected Mode.