diff --git a/src/compilerEs/fan/ast/JsExpr.fan b/src/compilerEs/fan/ast/JsExpr.fan
index a1847ca84..f3594fc7a 100644
--- a/src/compilerEs/fan/ast/JsExpr.fan
+++ b/src/compilerEs/fan/ast/JsExpr.fan
@@ -267,6 +267,19 @@ class JsExpr : JsNode
     leave    := x.leave
     isAssign := x.assignTarget != null
 
+    // convert binary expressions assigning or testing for equivalence to
+    // synthetic str "_once_" to undefined
+    if (lhs.expr is FieldExpr)
+    {
+      fe := (FieldExpr)lhs.expr
+      if (rhs.expr.id === ExprId.strLiteral && 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/JsType.fan b/src/compilerEs/fan/ast/JsType.fan
index 35ec6dbef..2a4912b35 100644
--- a/src/compilerEs/fan/ast/JsType.fan
+++ b/src/compilerEs/fan/ast/JsType.fan
@@ -235,6 +235,9 @@ class JsType : JsNode
 
   private static Str fieldDefVal(FieldDef f)
   {
+    // initialize once fields to undefined
+    if (f.isOnce) return "undefined"
+
     defVal    := "null"
     fieldType := f.fieldType
     if (!fieldType.isNullable)