Node Fundamentals: CLI and Repl

This is a series of posts that will illustrate the what, why and how of Node. I'll be sharing my learnings from a course on Advanced NodeJS by Samer Buna offered on PluralSight. Any code samples tagged or attached will be available at the following repo.

jscomplete / advanced-nodejs

For help, ask in #questions at slack.jscomplete.com

Node CLI and REPL

Node CLI comes with a variety of options to expose built-in debugging, multiple ways to execute scripts and other helpful runtime options.

Running a node command without any arguments starts a REPL.

R - Read
E - Eval
P - Print
L = Loop

When in REPL, you hit enter, it reads the command, executes it, prints the result and waits for the next command.

Helpful CLI Tips and Tricks

• -c - Syntax check
• -p - Print Command. e.g node -p "process.argv.slice(1) test 42" will print ['test', '42']

Helpful Repl Tricks and Tips

• Autocomplete by Tab

• rlwrap utility to track reverse search.
  

NODE_NOREADLINE=1 rlwrap node

• _ is used to capture the last evaluated value.

• Special commands that begin with a dot.

  • .help to print all such commands.
  • .break to break out of a multiline session.
  • .load to load external script file
  • .save to save current session

• You can create your own repl with custom options by requiring a repl module and starting it with custom options. You can also control repl's global context in case of preloading a library of data.

Example below will start the repl in strict mode and doesn't print anything when result is undefined. Also, it will have lodash available globally.

  const repl = require("repl");
  const lodash = require("lodash");

  let r = repl.start({
    ignoreUndefined: true,
    replMode: repl.REPL_STRICT_MODE
  });

  r.context.lodash = lodash;