diff --git a/src/compilerEs/fan/ast/JsExpr.fan b/src/compilerEs/fan/ast/JsExpr.fan
index a1847ca84..61c566752 100644
--- a/src/compilerEs/fan/ast/JsExpr.fan
+++ b/src/compilerEs/fan/ast/JsExpr.fan
@@ -267,6 +267,20 @@ class JsExpr : JsNode
     leave    := x.leave
     isAssign := x.assignTarget != null
 
+    // convert binary expressions testing for equivalence to
+    // synthetic str "_once_" to undefined
+    if (lhs.expr is FieldExpr)
+    {
+      fe := (FieldExpr)lhs.expr
+      if (rhs.expr.id === ExprId.strLiteral &&
+          symbol == "===" && rhs.expr->val == "_once_")
+      {
+        lhs.write
+        js.w(" ${symbol} undefined", loc)
+        return
+      }
+    }
+
     if (isAssign && lhs.expr is FieldExpr)
     {
       fe := (FieldExpr)lhs.expr
diff --git a/src/compilerEs/fan/ast/JsNode.fan b/src/compilerEs/fan/ast/JsNode.fan
index 3d010e3bb..d02b958c1 100644
--- a/src/compilerEs/fan/ast/JsNode.fan
+++ b/src/compilerEs/fan/ast/JsNode.fan
@@ -247,10 +247,11 @@ abstract class JsNode
     "sys::Str":     true
   ]
 
-  Void writeBlock(Block? block)
+  Void writeBlock(Block? block, |Stmt->Bool|? filter := null)
   {
     if (block == null) return
     block.stmts.each |stmt| {
+      if (filter?.call(stmt) ?: false) return
       writeStmt(stmt)
       js.wl(";")
     }
diff --git a/src/compilerEs/fan/ast/JsType.fan b/src/compilerEs/fan/ast/JsType.fan
index 35ec6dbef..79081770d 100644
--- a/src/compilerEs/fan/ast/JsType.fan
+++ b/src/compilerEs/fan/ast/JsType.fan
@@ -146,7 +146,7 @@ class JsType : JsNode
     if (instanceInit != null)
     {
       plugin.curMethod = instanceInit
-      writeBlock(instanceInit.code)
+      writeBlock(instanceInit.code) |stmt| { stmt.isOnceFieldInit ? true : false }
       plugin.curMethod = null
     }
     js.unindent
@@ -235,6 +235,9 @@ class JsType : JsNode
 
   private static Str fieldDefVal(FieldDef f)
   {
+    // once fields are initialized to undefined
+    if (f.isOnce) return "undefined"
+
     defVal    := "null"
     fieldType := f.fieldType
     if (!fieldType.isNullable)
@@ -269,6 +272,13 @@ class JsType : JsNode
     // we generate our own special version of this
     if (f.parent.isEnum && accessName == "vals") return
 
+    // special handling for static once fields
+    if (f.isOnce)
+    {
+      js.wl("static ${accessName}(it) { if (it === undefined) return ${target}.${privName}; else ${target}.${privName} = it; }").nl
+      return
+    }
+
     js.wl("static ${accessName}() {").indent
 
     fieldAccess := "${target}.${privName}"
@@ -388,7 +398,9 @@ class JsType : JsNode
       }
 
       // method body
-      writeBlock(m.code)
+      |Stmt s->Bool|? filter := null
+      if (m.isStatic) filter = |Stmt s->Bool| { s.isOnceFieldInit ? true : false }
+      writeBlock(m.code, filter)
     }
 
     js.unindent