what does `program.parse(process.argv)` do in commander.js?

Question

I want to user commander.js and inquirer.js to ask questions and collect the answer to create a User instance:

// index.js
const mongoose = require('mongoose');
const User = require('./model/user')
const {addUser,listAllUsers,findUserByEmail,updateUser,deleteUser} = require('./model_methods/user_methods')
const { program } = require('commander');
var inquirer = require('inquirer');

// connect to DB
const db = mongoose.connect('mongodb://localhost:27017/myImportantDates', {
    useNewUrlParser: true, 
    useUnifiedTopology: true, 
});

const questions = [
    {
      type: 'input',
      name: 'email',
      message: 'user email'
    },
    {
      type: 'input',
      name: 'name',
      message: 'user name'
    },
    {
      type: 'input',
      name: 'password',
      message: 'user password'
    },
  ];



program
   .version('0.0.1')
   .description('The important dates app');

program
   .command('add')
   .alias('a')
   .description('Add a user')
   .action(
       inquirer
         .prompt(questions)
         .then( answers => {
            addUser(answers)
          })
          .catch(err =>{
            console.log(error) 
          })
   )
program.parse(process.argv);

When I run it with node index.js add , the questions array ask one question and quit:

@DESKTOP-5920U38:/mnt/c/Users/myApp$ node index.js add
? user email 
@DESKTOP-5920U38:/mnt/c/Users/myApp$ 

When I delete program.parse(process.argv) , however, everything works fine, it can return me the new User instance.

I check the documents: https://github.com/tj/commander.js/ Still have no idea what happened. Does anybody know more about this??

What I found just now is that if I put program.parse(process.argv) in the beginning like this in the first set up of the program instance:

program
   .version('0.0.1')
   .description('The important dates app')
   .parse(process.argv);

It works too. But I still don't know why the order matters.

Answer 1

In Node.js, process.argv is an array containing the command line arguments passed when the Node.js process was launched . So, program.parse(process.argv) parses the the command line options for arguments, which is bypassing your inquierer.js prompt. You can leave it out.

Answer 2

program.parse(process.argv) is called after setting up your program. It takes the command line arguments ( process.argv ) and parses them using your declared program, displaying errors or calling action handlers etc.

Based on your example code, an issue is the action handler takes a function parameter. eg

.action(() => {
   // code goes here
});