Just to be clear, `deno eval "console.log(5)"` has existed for some time which is similar to `node -e "console.log(5)"`. In this case, the deno repl subcommand's --eval flag provides a cross-platform way to evaluate some setup code before launching into the REPL. This is useful if you alias the command in your shell's initialization file allowing you to easily launch a REPL with everything already setup to your liking.

For example:

  $ alias add_repl='deno repl --eval "const add = (a, b) => a + b"'
  ...
  $ add_repl
  Deno 1.13
  exit using ctrl+d or close()
  > add(1, 2)
  3

They wanted to keep feeding stdin that’s why. Their 2+2 is just a placeholder for something bigger, which could even be several lines of input that they wanted to prepend, followed by manual input.

In other words they are talking about initiating a REPL session with some initial input followed by manual input.

Node's `-p` option will print the result of the evaluation.

    $ node -p 1+1
    2