JS temp generation with array/seq/string issues

[metagn]

JS is failing at doing operations when modifying var Ts, because of incorrectly generated and used temp variables.

• inc/+= failures (dec probably fails the same way):

  • JS Backend generates invalid assignments on LHS #11331
  • [JS] Invalid codegen #12672

• Seq/string addition:

  • Failing seq/table operation JS codegen #13966 (the main problem in this issue is arrays with negative index but it would still break because a temp is being wrongly used)
  • Bad JS codegen when adding to string field in object in seq with complex expression for index #14087
  • JS: adding to uninitialized seq/string inside object variant is no-op #14117

• reset, probably related, works with many bugfixes for js #14158:

  • [JS] reset sets value types to null #12256

All of these use an index with a subscript first (except #14117, that accesses a field) then do the add/inc operation. They all assign to temp variables instead of a field/index, like if (null != (Temp1 = arr[i-1], Temp1)) { Temp1 += 1 } else { Temp1 = 1 } when it should be if (null != (Temp1 = i-1, Temp1)) { arr[Temp1] += 1 } else { arr[Temp1] = 1 } if anything.

From my deduction, the issue is either in maybeMakeTemp in jsgen, or the subscripts like arr[i-1] are not being recognized as var T by the compiler.