fix #8821 (#15809)

compiler/jsgen.nim

@@ -865,6 +865,7 @@ proc genRaiseStmt(p: PProc, n: PNode) =
 proc genCaseJS(p: PProc, n: PNode, r: var TCompRes) =
   var
     cond, stmt: TCompRes
+    totalRange = 0
   genLineDir(p, n)
   gen(p, n[0], cond)
   let stringSwitch = skipTypes(n[0].typ, abstractVar).kind == tyString
@@ -884,6 +885,10 @@ proc genCaseJS(p: PProc, n: PNode, r: var TCompRes) =
         let e = it[j]
         if e.kind == nkRange:
           var v = copyNode(e[0])
+          inc(totalRange, int(e[1].intVal - v.intVal))
+          if totalRange > 65535:
+            localError(p.config, n.info, 
+                       "Your case statement contains too many branches, consider using if/else instead!")
           while v.intVal <= e[1].intVal:
             gen(p, v, cond)
             lineF(p, "case $1:$n", [cond.rdLoc])

tests/js/t8821.nim

@@ -0,0 +1,12 @@
+discard """
+  errormsg: "Your case statement contains too many branches, consider using if/else instead!"
+"""
+
+proc isInt32(i: int): bool =
+  case i 
+  of 1 .. 70000:
+    return true
+  else:
+    return false
+
+discard isInt32(1)