Fix global destructor injection for JS backend (#19797)

* Fix global destructor injection for JS backend

* Moved global destructors injection before the final call to transform and
  generate JS code. It had previously been after and thus not no JS was
  generated for them.
* Added some internal documentation of `jsgen`.
* Enable a current destructor test to cover the JS backend as well.
* Fixes the JS aspect of #17237.

* Fixed global destructor injection order for JS backend

Co-authored-by: quantimnot <quantimnot@users.noreply.github.com>

compiler/jsgen.nim

@@ -2392,6 +2392,7 @@ proc optionalLine(p: Rope): Rope =
     return p & "\L"
 
 proc genProc(oldProc: PProc, prc: PSym): Rope =
+  ## Generate a JS procedure ('function').
   var
     resultSym: PSym
     a: TCompRes
@@ -2673,6 +2674,7 @@ proc gen(p: PProc, n: PNode, r: var TCompRes) =
   else: internalError(p.config, n.info, "gen: unknown node type: " & $n.kind)
 
 proc newModule(g: ModuleGraph; module: PSym): BModule =
+  ## Create a new JS backend module node.
   new(result)
   result.module = module
   result.sigConflicts = initCountTable[SigHash]()
@@ -2684,6 +2686,7 @@ proc newModule(g: ModuleGraph; module: PSym): BModule =
     PGlobals(g.backend).inSystem = true
 
 proc genHeader(): Rope =
+  ## Generate the JS header.
   result = rope("""/* Generated by the Nim Compiler v$1 */
     var framePtr = null;
     var excHandler = 0;
@@ -2714,6 +2717,8 @@ proc addHcrInitGuards(p: PProc, n: PNode,
     genStmt(p, n)
 
 proc genModule(p: PProc, n: PNode) =
+  ## Generate the JS module code.
+  ## Called for each top level node in a Nim module.
   if optStackTrace in p.options:
     p.body.add(frameCreate(p,
         makeJSString("module " & p.module.module.name.s),
@@ -2742,6 +2747,7 @@ proc genModule(p: PProc, n: PNode) =
     p.body.add(frameDestroy(p))
 
 proc myProcess(b: PPassContext, n: PNode): PNode =
+  ## Generate JS code for a node.
   result = n
   let m = BModule(b)
   if passes.skipCodegen(m.config, n): return n
@@ -2754,6 +2760,7 @@ proc myProcess(b: PPassContext, n: PNode): PNode =
   p.g.code.add(p.body)
 
 proc wholeCode(graph: ModuleGraph; m: BModule): Rope =
+  ## Combine source code from all nodes.
   let globals = PGlobals(graph.backend)
   for prc in globals.forwarded:
     if not globals.generatedSyms.containsOrIncl(prc.id):
@@ -2779,28 +2786,41 @@ proc getClassName(t: PType): Rope =
   else: result = rope(s.name.s)
 
 proc myClose(graph: ModuleGraph; b: PPassContext, n: PNode): PNode =
-  result = myProcess(b, n)
+  ## Finalize JS code generation of a Nim module.
+  ## Param `n` may contain nodes returned from the last module close call.
   var m = BModule(b)
   if sfMainModule in m.module.flags:
-    for destructorCall in graph.globalDestructors:
-      n.add destructorCall
+    # Add global destructors to the module.
+    # This must come before the last call to `myProcess`.
+    for i in countdown(high(graph.globalDestructors), 0):
+      n.add graph.globalDestructors[i]
+  # Process any nodes left over from the last call to `myClose`.
+  result = myProcess(b, n)
+  # Some codegen is different (such as no stacktraces; see `initProcOptions`)
+  # when `std/system` is being processed.
   if sfSystemModule in m.module.flags:
     PGlobals(graph.backend).inSystem = false
+  # Check if codegen should continue before any files are generated.
+  # It may bail early is if too many errors have been raised.
   if passes.skipCodegen(m.config, n): return n
+  # Nim modules are compiled into a single JS file.
+  # If this is the main module, then this is the final call to `myClose`.
   if sfMainModule in m.module.flags:
     var code = genHeader() & wholeCode(graph, m)
     let outFile = m.config.prepareToWriteOutput()
-
+    # Generate an optional source map.
     if optSourcemap in m.config.globalOptions:
       var map: SourceMap
       (code, map) = genSourceMap($(code), outFile.string)
       writeFile(outFile.string & ".map", $(%map))
+    # Check if the generated JS code matches the output file, or else
+    # write it to the file.
     if not equalsFile(code, outFile):
       if not writeRope(code, outFile):
         rawMessage(m.config, errCannotOpenFile, outFile.string)
 
-
 proc myOpen(graph: ModuleGraph; s: PSym; idgen: IdGenerator): PPassContext =
+  ## Create the JS backend pass context `BModule` for a Nim module.
   result = newModule(graph, s)
   result.idgen = idgen
 

tests/destructor/t5342.nim

@@ -1,5 +1,6 @@
 discard """
-  matrix: "--gc:refc; --gc:arc"
+  matrix: "--mm:refc; --mm:arc"
+  targets: "c js"
   output: '''
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