How to implement a negating flag

Scenario

You have a Boolean flag defined, e.g. --dev.

$ example --dev

You add dev: true to your config file making it the default state. Now, you no longer need to set --dev every time.

$ example

However, occasionally you might want to temporarity unset dev mode at the command line. You need a negating flag.

$ example --no-dev

Command-line-args does not have a built-in opinion on how a negating flag should be defined (to enable users to name options in their own language). Here is an example implementation.

Example

const commandLineArgs = require('command-line-args')
const optionDefinitions = [
  { name: 'dev', type: Boolean, defaultValue: true },
  { name: 'no-dev', type: Boolean }
]

const options = commandLineArgs(optionDefinitions)
const dev = options.dev && !options['no-dev']
console.log(`dev: ${dev}`)

Usage

$ node example.js
dev: true

$ node example.js --dev
dev: true

$ node example.js --dev --no-dev
dev: false

$ node example.js --no-dev
dev: false

Displaying a usage guide

You might not want to list --no-dev as a separate option in the usage guide. If using command-line-usage, you can hide options in an optionList section using options.hide.

Here's a full example.

const commandLineArgs = require('command-line-args')
const optionDefinitions = [
  {
    name: 'dev',
    type: Boolean,
    defaultValue: true,
    description: 'Set to enable dev mode. If already enabled, set --no-dev to disable.'
  },
  { name: 'no-dev', type: Boolean },
  {
    name: 'help',
    type: Boolean,
    description: 'Display a usage guide.'
  }
]

const options = commandLineArgs(optionDefinitions)
const dev = options.dev && !options['no-dev']

if (options.help) {
  const commandLineUsage = require('command-line-usage')
  const usage = commandLineUsage([
    {
      header: 'Options',
      optionList: optionDefinitions,
      hide: ['no-dev']
    }
  ])
  console.log(usage)
}

Usage

The --no-dev option is defined but not displayed.

$ node example.js --help

Options

  --dev     Set to enable dev mode. If already enabled, set --no-dev to disable.
  --help    Display a usage guide.