diff --git a/nix-js/runtime-ts/src/builtins/index.ts b/nix-js/runtime-ts/src/builtins/index.ts
index 129189b..786808d 100644
--- a/nix-js/runtime-ts/src/builtins/index.ts
+++ b/nix-js/runtime-ts/src/builtins/index.ts
@@ -249,7 +249,7 @@ export const builtins: any = {
 
   builtins: createThunk(() => builtins),
   currentSystem: createThunk(() => {
-    throw new Error("Not implemented: currentSystem");
+    return "x86_64-linux"
   }),
   currentTime: createThunk(() => Date.now()),
 
diff --git a/nix-js/runtime-ts/src/builtins/misc.ts b/nix-js/runtime-ts/src/builtins/misc.ts
index a7aa403..da9d350 100644
--- a/nix-js/runtime-ts/src/builtins/misc.ts
+++ b/nix-js/runtime-ts/src/builtins/misc.ts
@@ -4,7 +4,8 @@
 
 import { force } from "../thunk";
 import { CatchableError } from "../types";
-import type { NixBool, NixStrictValue, NixValue, NixString } from "../types";
+import type { NixBool, NixStrictValue, NixValue } from "../types";
+import { forceList, forceAttrs, forceFunction } from "../type-assert";
 import * as context from "./context";
 
 export const addErrorContext =
@@ -138,6 +139,34 @@ export const tryEval = (e: NixValue): { success: NixBool; value: NixStrictValue
 
 export const zipAttrsWith =
   (f: NixValue) =>
-  (list: NixValue): never => {
-    throw new Error("Not implemented: zipAttrsWith");
+  (list: NixValue): NixValue => {
+    const listForced = forceList(list);
+
+    // Map to collect all values for each attribute name
+    const attrMap = new Map<string, NixValue[]>();
+
+    // Iterate through each attribute set in the list
+    for (const item of listForced) {
+      const attrs = forceAttrs(force(item) as NixValue);
+
+      // Collect all attribute names and their values
+      for (const [key, value] of Object.entries(attrs)) {
+        if (!attrMap.has(key)) {
+          attrMap.set(key, []);
+        }
+        attrMap.get(key)!.push(value);
+      }
+    }
+
+    // Build the result attribute set
+    const result: Record<string, NixValue> = {};
+
+    for (const [name, values] of attrMap.entries()) {
+      // Apply f to name and values list
+      // f is curried: f name values
+      const fWithName = forceFunction(f)(name);
+      result[name] = forceFunction(fWithName)(values);
+    }
+
+    return result;
   };
diff --git a/nix-js/runtime-ts/src/helpers.ts b/nix-js/runtime-ts/src/helpers.ts
index a8b5070..0bbc8e0 100644
--- a/nix-js/runtime-ts/src/helpers.ts
+++ b/nix-js/runtime-ts/src/helpers.ts
@@ -72,15 +72,15 @@ export const select = (obj: NixValue, key: NixValue): NixValue => {
  *
  * @param obj - Attribute set to select from
  * @param key - Key to select
- * @param default_val - Value to return if key not found
- * @returns obj[key] if exists, otherwise default_val
+ * @param default_val - Value to return if key not found (will be forced if it's a thunk)
+ * @returns obj[key] if exists, otherwise force(default_val)
  */
 export const selectWithDefault = (obj: NixValue, key: NixValue, default_val: NixValue): NixValue => {
   const attrs = forceAttrs(obj);
   const forced_key = forceString(key);
 
   if (!(forced_key in attrs)) {
-    return default_val;
+    return force(default_val);
   }
 
   return attrs[forced_key];
diff --git a/nix-js/src/fetcher/mod.rs b/nix-js/src/fetcher.rs
similarity index 99%
rename from nix-js/src/fetcher/mod.rs
rename to nix-js/src/fetcher.rs
index b00bff4..87663ce 100644
--- a/nix-js/src/fetcher/mod.rs
+++ b/nix-js/src/fetcher.rs
@@ -8,8 +8,6 @@ mod nar;
 pub use cache::FetcherCache;
 pub use download::Downloader;
 
-use std::path::PathBuf;
-
 use deno_core::op2;
 use serde::Serialize;
 
diff --git a/nix-js/src/ir/downgrade.rs b/nix-js/src/ir/downgrade.rs
index 551d0df..64d210a 100644
--- a/nix-js/src/ir/downgrade.rs
+++ b/nix-js/src/ir/downgrade.rs
@@ -245,7 +245,8 @@ impl<Ctx: DowngradeContext> Downgrade<Ctx> for ast::Select {
         let expr = self.expr().unwrap().downgrade(ctx)?;
         let attrpath = downgrade_attrpath(self.attrpath().unwrap(), ctx)?;
         let default = if let Some(default) = self.default_expr() {
-            Some(default.downgrade(ctx)?)
+            let default_expr = default.downgrade(ctx)?;
+            Some(ctx.new_expr(Ir::Thunk(default_expr)))
         } else {
             None
         };
diff --git a/nix-js/tests/operators.rs b/nix-js/tests/operators.rs
index 32dd2cd..4c2e45f 100644
--- a/nix-js/tests/operators.rs
+++ b/nix-js/tests/operators.rs
@@ -99,3 +99,21 @@ fn logical_not() {
     assert_eq!(eval("!true"), Value::Bool(false));
     assert_eq!(eval("!false"), Value::Bool(true));
 }
+
+#[test]
+fn select_with_default_lazy_evaluation() {
+    assert_eq!(eval("{ a = 1; }.a or (1 / 0)"), Value::Int(1));
+}
+
+#[test]
+fn select_with_default_nested_lazy() {
+    assert_eq!(
+        eval("{ a.b = 42; }.a.b or (builtins.abort \"should not evaluate\")"),
+        Value::Int(42)
+    );
+}
+
+#[test]
+fn select_with_default_fallback() {
+    assert_eq!(eval("{ a = 1; }.b or 999"), Value::Int(999));
+}