An example of making custom syntax extensions for GMEdit. This collapses `//hello` in a file to `//hi` in editor and back.

config.json

{
	"name": "synext",
	"description": "",
	"scripts": ["synext.js"],
	"stylesheets": []
}

synext.js

(function() {
	const SyntaxExtension = $gmedit["synext.SyntaxExtension"];
	function MySynExt() {
		SyntaxExtension.call(this, "mySynExt", "My syntax extension");
	}
	MySynExt.prototype = GMEdit.extend(SyntaxExtension.prototype, {
		preproc: function(codeEditor, gmlCode) {
			return gmlCode.split("//hello").join("//hi");
		},
		postproc: function(codeEditor, gmlCode) {
			return gmlCode.split("//hi").join("//hello");
		},
	});
	function init() {
		$gmedit["file.kind.KGml"].syntaxExtensions.push(new MySynExt());
	}
	//
	GMEdit.register("synext", { init: init });
})();