fixes #21209; exclude threads options for JS (#21233)

compiler/commands.nim

@@ -649,6 +649,8 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
   of "backend", "b":
     let backend = parseEnum(arg.normalize, TBackend.default)
     if backend == TBackend.default: localError(conf, info, "invalid backend: '$1'" % arg)
+    if backend == backendJs: # bug #21209
+      conf.globalOptions.excl {optThreadAnalysis, optThreads}
     conf.backend = backend
   of "doccmd": conf.docCmd = arg
   of "define", "d":

tests/js/t21209.nim

@@ -0,0 +1,6 @@
+discard """
+  action: "compile"
+  cmd: "nim check --warning[UnusedImport]:off $file"
+"""
+
+import std/times

tests/js/t21209.nims

@@ -0,0 +1 @@
+--b:js