Default value of `false` doesn't work for `narg: { flag: 0 }`

yargs-parser doesn't seem to work when a boolean flag is defaulted to false and then activated from the args string. For example:

const YargsParser = require('yargs-parser');

YargsParser('-a hello -b world', {
    narg: {
        a: 1,
        b: 0,
    },
    default: {
        b: false,
    },
    alias: {
        blah: [ 'b', 'bl' ],
    },
});


// expected output
{
    _: ['world'],
    a: 'hello',
    blah: true,
    b: true,
    bl: true
}


// actual output
{
    _: ['world'],
    a: 'hello',
    blah: false,
    b: false,
    bl: false
}

If I explicitly marked it as boolean: true, it seems to work as expected. However, the problem is if the option/yargs-parser configuration had boolean replaced with narg to allow the flag value to be of multiple types (falling back to a boolean if no value given), then setting it solely as boolean will break the functionality of the other values it could be.

Admittedly, parsedArgs.b would be undefined if it weren't defaulted which is still falsey, but this still came as a surprise to me since narg: { blah: 0 } should be enough to mark it as a boolean. It's not the end of the world but would be nice to not have this gotcha present in the parser that breaks if the user happens to add a default value of false.

Related

• Issue 256.
• Issue 216.
• PR 191.

Start by reproducing the shown YargsParser invocation with narg, default, and alias options, then trace how the parser handles a flag activated from the args string when its default is false. Done means the example returns true for b, blah, and bl while preserving support for other narg values.