https://github.com/nim-lang/Nim/pull/15826/files#r585368355 (#17233)

lib/pure/includes/osenv.nim

@@ -5,9 +5,10 @@ when not declared(os) and not declared(ospaths):
 
 when defined(nodejs):
   proc getEnv*(key: string, default = ""): string {.tags: [ReadEnvEffect].} =
-    var ret: cstring
+    var ret = default.cstring
     let key2 = key.cstring
-    {.emit: "`ret` = process.env[`key2`];".}
+    {.emit: "const value = process.env[`key2`];".}
+    {.emit: "if (value !== undefined) { `ret` = value };".}
     result = $ret
 
   proc existsEnv*(key: string): bool {.tags: [ReadEnvEffect].} =