Example of Optimized Javascript Object as a StringMap<Dynamic>

JSObjectStringMap.hx

package project;

abstract JSObjectStringMap({}) {
    public inline function new(?i:{}) {
        this = if (i == null) {} else i;
    }
}

JSObjectStringMapTools.hx

package starlight.core;

import haxe.macro.Expr;
import haxe.macro.ExprTools;
import starlight.core.JSObjectStringMap;

class JSObjectStringMapTools {

    macro public static inline function set(obj:ExprOf<JSObjectStringMap>, field:ExprOf<String>, value:Expr) {
        return macro untyped __js__('${ExprTools.toString(obj)}[${ExprTools.toString(field)}] = ${ExprTools.toString(value)}');
    }

    macro public static inline function get(obj:ExprOf<JSObjectStringMap>, field:ExprOf<String>) {
        return macro untyped __js__('${ExprTools.toString(obj)}[${ExprTools.toString(field)}]');
    }

    macro public static inline function exists(obj:ExprOf<JSObjectStringMap>, field:ExprOf<String>) {
        return macro untyped __js__('${ExprTools.toString(field)} in ${ExprTools.toString(obj)}');
    }

    macro public static inline function keys(obj:ExprOf<JSObjectStringMap>) {
        return macro (untyped __js__('Object.keys(${ExprTools.toString(obj)})'):Array<String>);
    }

    macro public static inline function arrayKeys(obj:ExprOf<JSObjectStringMap>) {
        return macro (untyped __js__('Object.keys(${ExprTools.toString(obj)})'):Array<String>);
    }

    macro public static inline function size(obj:ExprOf<JSObjectStringMap>) {
        return macro untyped __js__('Object.keys(${ExprTools.toString(obj)}).length');
    }

    macro public static function values(obj:ExprOf<JSObjectStringMap>):ExprOf<Array<Dynamic>> {
        return macro [for (key in (untyped __js__('Object.keys(${ExprTools.toString(obj)})'):Array<String>)) (untyped __js__('${ExprTools.toString(obj)}[key]'))];
    }
}

Main.hx

package project;

import JSObjectStringMap;
import JSObjectStringMapTools;

class App extends Lens {
    static function main() {
        var a = new JSObjectStringMap();        //  compiles to: var a = { };
        JSObjectStringMapTools.set(a, 'a', 1);  //  compiles to: a['a'] = 1;
        JSObjectStringMapTools.keys(a);         //  compiles to: Object.keys(a);
    }
}

I'm leaving this here for reference from the mailinglist. I recommend using DynamicAccess in the standard library instead.