First we'll require through2:

const through = require('through2') 

Next we'll use it to create a stream that uppercases incoming data:

const upper = through((chunk, enc, cb) => { 
  cb(null, chunk.toString().toUpperCase()) 
}) 

Finally, we'll create a pipeline from the terminal's STDIN through our upper stream to the terminal's STDOUT:

process.stdin.pipe(upper).pipe(process.stdout) 

Now if we start our program:

$ node index.js 

Each line we type into the terminal will be uppercased, as demonstrated in the following image: