fix for js strict mode (#18799)

* Assignments, which would accidentally create global
variables, instead throw an error in strict mode

* Assignment to a getter-only property

Co-authored-by: Sven Keller <s.keller@cortona.de>

compiler/jsgen.nim

@@ -1769,7 +1769,7 @@ proc genVarInit(p: PProc, v: PSym, n: PNode) =
       inc p.extraIndent
     elif useGlobalPragmas:
       lineF(p, "if (globalThis.$1 === undefined) {$n", varName)
-      varCode = $varName
+      varCode = "globalThis." & $varName
       inc p.extraIndent
     else:
       varCode = "var $2"

lib/system/jssys.nim

@@ -593,12 +593,9 @@ proc nimCopy(dest, src: JSRef, ti: PNimType): JSRef =
         `result` = null;
       }
       else {
-        if (`dest` === null || `dest` === undefined) {
+        if (`dest` === null || `dest` === undefined || `dest`.length != `src`.length) {
           `dest` = new Array(`src`.length);
         }
-        else {
-          `dest`.length = `src`.length;
-        }
         `result` = `dest`;
         for (var i = 0; i < `src`.length; ++i) {
           `result`[i] = nimCopy(`result`[i], `src`[i], `ti`.base);