JS backend properly extends string with setLen (#21087)

* Add test case

* Extend string with '0' when setting length to be longer

compiler/jsgen.nim

@@ -2209,7 +2209,9 @@ proc genMagic(p: PProc, n: PNode, r: var TCompRes) =
       if optOverflowCheck notin p.options: binaryExpr(p, n, r, "", "$1 -= $2")
       else: binaryExpr(p, n, r, "subInt", "$1 = subInt($3, $2)", true)
   of mSetLengthStr:
-    binaryExpr(p, n, r, "mnewString", "($1.length = $2)")
+    binaryExpr(p, n, r, "mnewString",
+      """if ($1.length < $2) { for (var i = $3.length; i < $4; ++i) $3.push(0); }
+         else {$3.length = $4; }""")
   of mSetLengthSeq:
     var x, y: TCompRes
     gen(p, n[1], x)

tests/js/t20235.nim

@@ -0,0 +1,11 @@
+discard """
+  action: "run"
+  output: "0 4"
+"""
+
+proc main =
+  var s = ""
+  s.setLen(4)
+  echo s[0].ord, " ", s.len
+
+main()