How can I provide a variable percentage to mapshaper via the cli?

I need to simplify Admin 0 – Details > map units 10m NaturalEarthData ( file ). It is 25mb and I would like to get it down to around 1mb or less. This should be easy because the 110m near-equivalent data is ~1mb (it is missing features I need).

mapshaper looks like it should do the job.

Trying the command:

mapshaper ne_10m_admin_0_map_units.geo.json -simplify 1% keep-shapes -o simplified.geo.json

produces

for bonaire. The original data looks like:

If I increase the percentage, to 12, I get:

which is good enough, but now other polygons contain more detail than what I need, increasing the file size.

What I would like to be able to do is have more control over the percentage or something so that I can keep the file small. Small areas, like Bonaire, would have 12 applied and everyone else would be reduced to 5 or something.

According to the simplify documentation:

> variable Apply a variable amount of simplification to the paths in a polygon or polygon layer. This flag changes the interval=, percentage= and resolution= options to accept JavaScript expressions instead of literal values. (See the -each command for information on mapshaper JS expressions).

The each command implies that there is be a this object available with the information I need to control the percentage.

To test, creating the javascript file:

function percentage() {
	console.log( this )

	return 0.01
}

exports.percentage = percentage

and executed the command:

mapshaper ne_10m_admin_0_map_units.geo.json -require js_expression.js alias=_ -simplify variable percentage='_.percentage()' weighted keep-shapes -o simplified.geo.json

My function is used and the console.logging of this works, but it outputs:

{ percentage: [Function: percentage] }

and not the this that the each documentation discusses.

Is there a way to use mapshaper to control the percentage? If so, what have I missed?

Is there another way to use mapshaper to do what I need it to do?

The key part that was missed is that this needs to be passed to the function on the command line. It is not automatically passed.

Calling mapshaper with:

mapshaper ne_10m_admin_0_map_units.geo.json -require js_expression.js alias=_ -simplify variable percentage='_.percentage(this)' weighted keep-shapes -o simplified.geo.json

and then writing the javascript code as:

function percentage(a) {
    console.log( a.area )

    return 0.01
}

exports.percentage = percentage

works and will output the area of the features processed. The value returned can be anything required.