flags without argument ('empty flags') are hard to use as boolean flag

[scarf]

Summary

• option("--empty-flag", "boolean switch") may cause surprising behavior with default value.
• while there's negatable flag, it may be preferable to not provide one for conciseness.

case 1: empty flag cannot be used as boolean

const boolToStr = (b: boolean) => b ? "true" : "false"

const test1 = await new Command()
  .option("--bool-value", "boolean switch")
  .action(({ boolValue }) => console.log(boolToStr(boolValue))) // type error
  .parse(["--bool-value"])
// boolValue: true | undefined

case 2: default option causes footgun behavior (always evaluates to false)

const { options } = await new Command()
  .option("--bool-value", "boolean switch", { default: false })
  .parse(["--bool-value"])
// boolValue: boolean

however, boolValue always evaluates to false regardless of whether flag is provided.

const { options } = await new Command()
  .option("--bool-value", "boolean switch", { value: (x) => !!x, default: false })
  .parse(Deno.args)

console.log({ options })

similarly, default property completely overrides value mapper.

case 3: default value can be only used for action params

const test3 = await new Command()
  .option("--bool-value", "boolean switch")
  .action(({ boolValue = false }) => console.log(boolValue))
// boolValue: boolean

Possible Solutions?

• document the behavior
• making default: false overridable for empty flags

[Benjamin Fischer]

case 1: empty flag cannot be used as boolean

Yeah, i noticed this as well. Maybe we should set the default value by default to false for boolean flags with no or with optional value.

case 2: default option causes footgun behavior (always evaluates to false)

This was a bug and is already fixed. Will be available in the next release.

[scarf]

that's good to know. would i make a PR for case 1?

[Benjamin Fischer]

I think if we adjust the boolean type to always be a boolean, we should also adjust collectible options to always be an array.
Feel free to open a PR, but the typings for the command module are a bit complex. Let me know if you need help!