To develop an Express.js application using TypeScript, you need to follow these steps:

1. Install TypeScript: In the command-line tool or terminal, navigate to your project directory and run the following command:

   npm install typescript --save-dev
   ```

   This will install the latest version of TypeScript and all its dependencies.

2. Initialize TypeScript: In the command-line tool or terminal, navigate to your project directory and run the following command:

npx tsc –init

   This will create a `tsconfig.json` file in your project directory, which contains configuration options for the TypeScript compiler.

3. Install Express and related TypeScript types: In the command-line tool or terminal, navigate to your project directory and run the following command:

`
npm install express @types/express –save

   This will install Express and related TypeScript types.

4. Create a TypeScript file: In your project directory, create an `app.ts` file and add the following code:

`typescript
import express from ‘express’;

const app = express();

app.get(‘/’, (req, res) => {
res.send(‘Hello, Express!’);
});

app.listen(3000, () => {
console.log(‘Server started on port 3000’);
});

   This will create a simple Express application that returns the message "Hello, Express!" at the root path.

5. Compile the TypeScript file: In the command-line tool or terminal, navigate to your project directory and run the following command:

`
npx tsc

   This will compile your TypeScript file and generate a JavaScript file.

6. Run your application: In the command-line tool or terminal, navigate to the directory containing your JavaScript file and run the following command:

`
node app.js

npx tsc && node app.js
“`

This will start your application and output the message “Server started on port 3000” in the command-line tool or terminal.

In summary, to develop an Express.js application using TypeScript, you need to install TypeScript, initialize TypeScript, install Express and related TypeScript types, create a TypeScript file, compile the TypeScript file, and then run your application.