diff --git a/src/build/fan/BuildPod.fan b/src/build/fan/BuildPod.fan
index 7e9a855c6..d563a15ec 100644
--- a/src/build/fan/BuildPod.fan
+++ b/src/build/fan/BuildPod.fan
@@ -227,7 +227,7 @@ abstract class BuildPod : BuildScript
     }
   }
 
-  @NoDoc protected CompilerInput stdFanCompilerInput()
+  @NoDoc protected virtual CompilerInput stdFanCompilerInput()
   {
     // add my own meta
     meta := this.meta.dup
diff --git a/src/compiler/fan/CompilerInput.fan b/src/compiler/fan/CompilerInput.fan
index d5e15bbf0..8266701b7 100644
--- a/src/compiler/fan/CompilerInput.fan
+++ b/src/compiler/fan/CompilerInput.fan
@@ -38,12 +38,18 @@ class CompilerInput
   **
   Bool isScript := false
 
-
   **
   ** Flag indicating if we should treat all types as having the @Js facet
   **
   Bool forceJs := false
 
+
+  **
+  ** Flag to force Fantom closures to be compiled with type reflection information
+  ** when emitting JavaScript.
+  **
+  Bool jsReflectClosures := false
+
   **
   ** Version to include in ouput pod's manifest.
   **
diff --git a/src/compilerEs/fan/CompileEsPlugin.fan b/src/compilerEs/fan/CompileEsPlugin.fan
index 1bb9055d1..f410e6ba5 100644
--- a/src/compilerEs/fan/CompileEsPlugin.fan
+++ b/src/compilerEs/fan/CompileEsPlugin.fan
@@ -23,6 +23,7 @@ class CompileEsPlugin : CompilerStep
   {
     this.sourcemap = SourceMap(this)
     this.js = JsWriter(buf.out, sourcemap)
+    this.closureSupport = JsClosure(this)
     pod.depends.each |depend| { dependOnNames[depend.name] = true }
     readJsProps
   }
@@ -45,6 +46,8 @@ class CompileEsPlugin : CompilerStep
 
   [Str:Bool] dependOnNames := [:] { def = false }
 
+  JsClosure closureSupport { private set }
+
 //////////////////////////////////////////////////////////////////////////
 // js.props
 //////////////////////////////////////////////////////////////////////////
diff --git a/src/compilerEs/fan/ast/JsClosure.fan b/src/compilerEs/fan/ast/JsClosure.fan
new file mode 100644
index 000000000..54e90ea6f
--- /dev/null
+++ b/src/compilerEs/fan/ast/JsClosure.fan
@@ -0,0 +1,107 @@
+//
+// Copyright (c) 2024, Brian Frank and Andy Frank
+// Licensed under the Academic Free License version 3.0
+//
+// History:
+//   15 Mar 2024  Matthew Giannini Creation
+//
+
+using compiler
+
+**
+** JsClosure provides utilities for working with JS closures.
+**
+class JsClosure : JsNode
+{
+  new make(CompilerSupport s) : super(s)
+  {
+  }
+
+  Bool emitReflection() { c.input.jsReflectClosures }
+
+  ** Write the actual ClosureExpr
+  Void writeClosure(ClosureExpr ce)
+  {
+
+    CType[] sigTypes := [,].addAll(ce.signature.params).add(ce.signature.ret)
+    isJs := sigTypes.all { !it.isForeign && checkJsSafety(it, loc) }
+    if (isJs)
+    {
+      if (emitReflection)
+      {
+        js.wl("sys.Func.__reflect(").indent
+        js.wl("${mapFuncSpec(ce)},", loc)
+      }
+
+      js.w("${methodParams(ce.doCall.params)}", loc).wl(" => {")
+      js.indent
+      old := plugin.thisName
+      plugin.thisName = "this\$"
+      writeBlock(ce.doCall.code)
+      plugin.thisName = old
+      js.unindent
+      js.w("}")
+
+      if (emitReflection)
+      {
+        js.w(")").unindent
+      }
+    }
+    else
+    {
+      // this closure uses non-JS types. Write a closure that documents this fact
+      js.wl("() => {")
+      js.wl("  // Cannot write closure. Signature uses non-JS types: ${ce.signature}")
+      js.wl("  throw sys.UnsupportedErr.make('Closure uses non-JS types: ' + ${ce.signature.toStr.toCode});")
+      js.w("}")
+    }
+  }
+
+  ** Write the unique closure specification fields for this pod (JsPod)
+  override Void write()
+  {
+    varToFunc.each |MethodDef func, Str var|
+    {
+      loc := func.loc
+      js.w("const ${var} = [${qnameToJs(func.ret)}.type\$,")
+      js.w("sys.List.make(sys.Param.type\$, [")
+      func.params.each |p,i|
+      {
+        if (i>0) js.w(",")
+        js.w("new sys.Param(${p.name.toCode}, ${p.paramType.signature.toCode}, ${p.hasDefault})")
+      }
+      js.w("])")
+        .w("];").nl
+    }
+    js.nl
+  }
+
+  private Str mapFuncSpec(ClosureExpr ce)
+  {
+    var := specKeyToVar.getOrAdd(specKey(ce)) |->Str|
+    {
+      // "${ce.enclosingType.pod}.__clos${plugin.nextUid}"
+      "__clos${plugin.nextUid}"
+    }
+    varToFunc[var] = ce.doCall
+    return var
+  }
+
+  private static Str specKey(ClosureExpr ce)
+  {
+    MethodDef func := ce.doCall
+    buf := StrBuf()
+    func.params.each |p|
+    {
+      buf.add("${p.name}-${p.paramType.signature}-${p.hasDefault},")
+    }
+    buf.add("${func.ret.signature}")
+    return buf.toStr
+  }
+
+  ** Func spec key to field variable name
+  private Str:Str specKeyToVar := [:]
+
+  ** Func spec field variable name to prototype function (for params and return type)
+  private Str:MethodDef varToFunc := [:] { ordered = true }
+}
\ No newline at end of file
diff --git a/src/compilerEs/fan/ast/JsExpr.fan b/src/compilerEs/fan/ast/JsExpr.fan
index 53e268114..a1847ca84 100644
--- a/src/compilerEs/fan/ast/JsExpr.fan
+++ b/src/compilerEs/fan/ast/JsExpr.fan
@@ -461,27 +461,7 @@ class JsExpr : JsNode
 
   private Void writeClosure(ClosureExpr ce)
   {
-    CType[] sigTypes := [,].addAll(ce.signature.params).add(ce.signature.ret)
-    isJs := sigTypes.all { !it.isForeign && checkJsSafety(it, loc) }
-    if (isJs)
-    {
-      js.w("${methodParams(ce.doCall.params)}", loc).wl(" => {")
-      js.indent
-      old := plugin.thisName
-      plugin.thisName = "this\$"
-      writeBlock(ce.doCall.code)
-      plugin.thisName = old
-      js.unindent
-      js.w("}")
-    }
-    else
-    {
-      // this closure uses non-JS types. Write a closure that documents this fact
-      js.wl("() => {")
-      js.wl("  // Cannot write closure. Signature uses non-JS types: ${ce.signature}")
-      js.wl("  throw sys.UnsupportedErr.make('Closure uses non-JS types: ' + ${ce.signature.toStr.toCode});")
-      js.w("}")
-    }
+    plugin.closureSupport.writeClosure(ce)
   }
 
 //////////////////////////////////////////////////////////////////////////
diff --git a/src/compilerEs/fan/ast/JsPod.fan b/src/compilerEs/fan/ast/JsPod.fan
index b3ac49228..debcbd5fc 100644
--- a/src/compilerEs/fan/ast/JsPod.fan
+++ b/src/compilerEs/fan/ast/JsPod.fan
@@ -47,6 +47,7 @@ class JsPod : JsNode
     writeTypes
     writeTypeInfo
     writeProps
+    writeClosureFields
     writeNatives
     writeExports
     js.wl("}).call(this);")
@@ -248,6 +249,11 @@ class JsPod : JsNode
     }
   }
 
+  private Void writeClosureFields()
+  {
+    plugin.closureSupport.write
+  }
+
   private Void writeNatives()
   {
     natives.each |f| { js.minify(f.in) }
diff --git a/src/sys/es/fan/Func.js b/src/sys/es/fan/Func.js
index cb3c4bf72..d469adc95 100644
--- a/src/sys/es/fan/Func.js
+++ b/src/sys/es/fan/Func.js
@@ -46,6 +46,8 @@ class Func extends Obj {
   static callList(f, args) { return f.__method.callList(args); }
 
   static params(f) {
+    if (f.__params) return f.__params;
+
     let mparams = f.__method.params();
     let fparams = mparams;
     if ((f.__method.flags$() & (FConst.Static | FConst.Ctor)) == 0) {
@@ -67,6 +69,13 @@ class Func extends Obj {
 
   static toStr(f) { return "sys::Func"; }
 
+  static __reflect(spec, f)
+  {
+    f.__returns = spec[0];
+    f.__params = spec[1];
+    return f;
+  }
+
   // TODO:FIXIT
   // retype(t) {
   //   t = t.toNonNullable();