Exports a json with all the markers in the selected compositions in After Effects. (exporting only the timestamp, but can be easily modified to export the others properties from the marker)

ae_marker_exporter.jsx

var jsonText = '{"markers":[';
var length = app.project.selection.length;

for(var i = 0; i < length;i++){
	var activeItem = app.project.selection[i];
	var tempNull = activeItem.layers.addNull(activeItem.duration);
	var tempPos = tempNull.property("ADBE Transform Group").property("ADBE Position");
	 
	tempPos.expression = "x = thisComp.marker.numKeys;[x,0];";
	var result = tempPos.value[0];

	jsonText += '{ "id":"' +activeItem.name+ '", "hits": [' ;
	 
	for (x = 1; x <= result; x++) {
		tempPos.expression = "x = thisComp.marker.key(" + x + ").time;[x,0];";
		jsonText += tempPos.value[0];
		if(x != result) { jsonText += ','; }
	}

	jsonText += ']}'

	if(i != length-1){ jsonText += ','; }

	 
}

jsonText += ']}'
var name = 'markers';
var file = new File("./" + name + ".json");
file.remove();
file.open("w", "TEXT");
file.lineFeed = "\n";
file.write(jsonText);
file.close();


tempNull.remove();