Home

Using node as a program entrypoint

Node has comprehensive docs on working as a command-line tool.

Here's just a few highlights to get the ball rolling:

see input options

Use node, itself, to get a glimpse into what it expects from the command line.

node --help

The output that appears in the terminal start like this,

Usage: node [options] [ script.js ] [arguments]
  node inspect [options] [ script.js | host:port ] [arguments]

Explaining a few things:

Expected input order

The node cli (Command-line interface) expects the order or input words to look something like

  1. node: the keyword that tells the machine to run the node runtime environment
  2. [options]: node will accept a bunch of options as the next arg - some examples below, maybe some of the first options to consider learning & working with?!
  3. [script.js] - a file that can be interpreted as a program run by node

CLI examples using flags and files

see v8 input options

node --v8-options

This will show a BUNCH of options that can be used to interact with the v8 js engine, perhaps one level "below" node itself.

evaluate the syntax of an argument

node -e "const a = 'horse'"

Running the above will return... nothing... which means the string is functional javascript!

node -e "cons = 'horse'"

Something like the above could be a typo - instead of const there is a cons.
Running the above will return something like...

[eval]:1
cons a = 'horse'
     ^

SyntaxError: Unexpected identifier
    at new Script (node:vm:100:7)
    at createScript (node:vm:257:10)
    at Object.runInThisContext (node:vm:305:10)
    at node:internal/process/execution:75:19
    at [eval]-wrapper:6:22
    at evalScript (node:internal/process/execution:74:60)
    at node:internal/main/eval_string:27:3

node...

  • sees the cons as the declaration of a variable placeholder referred to as cons
  • "looks for" the assignment of a value to the just-declared cons holder with something like a function name followed by (), or an equals sign...
  • Here, though, another variable a is present, which is not a node way of processing code
  • node cant handle this && throws a syntax error

check the syntax of a file

node --check programFile.js
node -c programFile.js
  • validate that the code 'parses':
    • for sensitive db-affecting code
    • for auto-generated code, maybe code written by others
    • on success: no output
    • on failure: error gets printed in output

checking a file with broken syntax

Node offers a -c and/or --check flag that performs a syntax check on the file without executing the code.
Here, a broken js syntax gets evaluated:

// broken.js
const wat = "This is a broken string
// notice the missing closing quote

# check the syntax of that file in a terminal using the "-c" flag
$ node -c a.js 
a.js:1
const a = "this is a broken string
          ^^^^^^^^^^^^^^^^^^^^^^^^

SyntaxError: Invalid or unexpected token
    at Object.compileFunction (node:vm:352:18)
    at wrapSafe (node:internal/modules/cjs/loader:1031:15)
    at checkSyntax (node:internal/main/check_syntax:66:3)
    at node:internal/main/check_syntax:39:3
$

evaluate or check

These two commands can seem like they do very similar things, -e for evaluate and -c for check.
Evaluate, though, will evaluate, or check, a string as a script.
Check, on the other hand, will check, or evaluate, a file/program.
Passing a file to evaluate will not work.
Passing a string to check will not work.

Tags:
NodeJS
Terminal
cmd