JScript example for JOSM ext_tools plugin (http://wiki.openstreetmap.org/wiki/JOSM/Plugins/ExtTools)

exttools_example.js

/* An example script for the JOSM ExtTools (ext_tools) plugin.
   http://wiki.openstreetmap.org/wiki/JOSM/Plugins/ExtTools
   After installing the ext_tools plugin, copy this script to your JOSM plugins/ext_tools.
   Find the External Tools preferences panel, and within, set a new tool with this CmdLine:
       cscript.exe //nologo exttools_example.js {lat} {lon} {PPD}
*/

var pixels=5;	//This number is only accurate for horizontal scaling; that's how PPD is calculated.

if (WScript.Arguments.length != 3) {
	WScript.StdErr.WriteLine("Syntax: " & WScript.ScriptName & " {lat} {lon} {PPD}" + vbLF
	  + WScript.Arguments.Count + " parameters provided, need 3!");
	WScript.Quit(3);
}
var lat = Number(WScript.Arguments(0));
var lon = Number(WScript.Arguments(1));
var ppd = Number(WScript.Arguments(2));

//WScript.StdErr.WriteLine("Parameters: lat='" + lat + "' lon='" + lon + "' PPD='" + ppd + "'");
var d = new Date();
var id = 0 - d.getHours() * 360000 - d.getMinutes() * 60000 - d.getSeconds() * 100 - d.getMilliseconds() * 10;	//Hope that, if editing continues past midnight, no seconds collision

function WriteNode(id, lat, lon, tags) {	//tags is a "|" delimited string with key=values
	WScript.StdOut.Write("<node id='" + id + "' lat='" + Coord(lat) + "' lon='" + Coord(lon) + "'");
	if (0 == tags.length) {
		WScript.StdOut.WriteLine(" />");
		return;
	}
	WScript.StdOut.WriteLine(">");
	a = tags.split("|");
	for (t in a) {
		WriteTag(a[t]);
	}
	WScript.StdOut.WriteLine("</node>");
}

function Coord(n) {
	return n.toFixed(7);	//OSM Node lat/lon has 7 decimal places; wiki.osm.org/wiki/Node
}

function WriteTag(tag) {	//Pass a "key=value" string; assumes more than one "=" is part of the value
//This assumes that any passed tag is already properly escaped (if needed) for XML use.
	var a = tag.split("=");
	WScript.StdOut.Write("  <tag k='" + a[0] + "'");
	WScript.StdOut.WriteLine(" v='" + a.slice(1).join("=") + "' />");
}


//Preamble
WScript.StdOut.WriteLine("<osm version='0.6'>");

//Put a node in the center, name it for debugging purposes.
WriteNode(id, lat, lon, "name=PPD=" + ppd.toString());

//Put nodes at offset extents
var offset = pixels / ppd / 100000;	//This was arrived at by trial and error; YMMV.
WriteNode(id-1, lat+offset, lon-offset, "");
WriteNode(id-2, lat+offset, lon+offset, "");
WriteNode(id-3, lat-offset, lon+offset, "");
WriteNode(id-4, lat-offset, lon-offset, "");

//Make a closed way out of the fudged extents.
WScript.StdOut.WriteLine("<way id='" + id + "'>");
for (var i = 1; i <= 4; i++) {
	WScript.StdOut.WriteLine("  <nd ref='" + (id - i).toString() + "' />");
}
WScript.StdOut.WriteLine("  <nd ref='" + (id - 1).toString() + "' />");
WriteTag("building=yes");
WScript.StdOut.WriteLine("</way>");

//Close out the XML
WScript.StdOut.WriteLine("</osm>");
WScript.Quit(0);

See also https://gist.github.com/goldfndr/5719016 for a nearly identical example written in VBScript. (No idea why search isn't bringing it up).