Optional argument to option with yargs

Question

I am trying to build a command-line interface with yargs, where one option takes an (optional!) argument:

const cli = yargs
.array('keyword')
.option('keyword', {
    alias: 'k',
    type: 'string',
    describe: 'add additional keyword or clear previous keywords without an argument'
)
.argv;

In other words the usage program --keyword --keyword=this --keyword=that is accepted.

How can I tell yargs to accept the option --keyword with or without an option?

Answer 1

It turns out that yargs will always accept empty arguments to options. The behavior differs depending on whether the option is an array option or not.

If you run programm --keyword --keyword=this --keyword=that and if you define your options like this:

const cli = yargs
.array('keyword')
.option('keyword', {
    alias: 'k',
    type: 'string',

})
.argv;
console.log(yargs)

You get this output:

{
  _: [],
  keyword: [ 'this', 'that' ],
  k: [ 'this', 'that' ],
  '$0': 'bin/program.js'
}

The option without an argument is simply ignored which is likely not what you want.

Without array:

const cli = yargs
.option('keyword', {
    alias: 'k',
    type: 'string',

})
.argv;
console.log(yargs)

You get this output:

{
  _: [],
  keyword: [ '', 'this', 'that' ],
  k: [ '', 'this', 'that' ],
  '$0': 'bin/program.js'
}

That means that the empty argument is saved in the result.