fix #17286 nim check -b:js works (#19704)

* fix #17286 nim check -b:js works

* fix

compiler/main.nim

@@ -65,6 +65,8 @@ proc commandCheck(graph: ModuleGraph) =
   if optWasNimscript in conf.globalOptions:
     defineSymbol(conf.symbols, "nimscript")
     defineSymbol(conf.symbols, "nimconfig")
+  elif conf.backend == backendJs:
+    setTarget(conf.target, osJS, cpuJS)
   semanticPasses(graph)  # use an empty backend for semantic checking only
   compileProject(graph)
 

tests/misc/t17286.nim

@@ -0,0 +1,16 @@
+discard """
+  cmd: "nim check -b:js $file"
+  action: "compile"
+"""
+
+# bug #17286
+
+import std/compilesettings
+
+static:
+  doAssert querySetting(backend) == "js"
+  doAssert defined(js)
+  doAssert not defined(c)
+
+import random
+randomize()