asyncjs: add then, catch for promise pipelining (#16871)

* asyncjs: add then
* improve tests, changelog, API
* fix cryptic windows error: The parameter is incorrect
* address comments

tests/js/tasync.nim

@@ -2,32 +2,76 @@ discard """
   output: '''
 x
 e
+done
 '''
 """
 
-import asyncjs
+#[
+xxx move this to tests/stdlib/tasyncjs.nim
+]#
 
-# demonstrate forward definition
-# for js
-proc y(e: int): Future[string] {.async.}
+import std/asyncjs
 
-proc e: int {.discardable.} =
-  echo "e"
-  return 2
+block:
+  # demonstrate forward definition for js
+  proc y(e: int): Future[string] {.async.}
 
-proc x(e: int): Future[void] {.async.} =
-  var s = await y(e)
-  if e > 2:
-    return
-  echo s
-  e()
+  proc e: int {.discardable.} =
+    echo "e"
+    return 2
 
-proc y(e: int): Future[string] {.async.} =
-  if e > 0:
-    return await y(0)
+  proc x(e: int): Future[void] {.async.} =
+    var s = await y(e)
+    if e > 2:
+      return
+    echo s
+    e()
+
+  proc y(e: int): Future[string] {.async.} =
+    if e > 0:
+      return await y(0)
+    else:
+      return "x"
+
+  discard x(2)
+
+import std/sugar
+from std/strutils import contains
+
+var witness: seq[string]
+
+proc fn(n: int): Future[int] {.async.} =
+  if n >= 7:
+    raise newException(ValueError, "foobar: " & $n)
+  if n > 0:
+    var ret = 1 + await fn(n-1)
+    witness.add $(n, ret)
+    return ret
   else:
-    return "x"
+    return 10
 
+proc main() {.async.} =
+  block: # then
+    let x = await fn(4)
+      .then((a: int) => a.float)
+      .then((a: float) => $a)
+    doAssert x == "14.0"
+    doAssert witness == @["(1, 11)", "(2, 12)", "(3, 13)", "(4, 14)"]
 
-discard x(2)
+    doAssert (await fn(2)) == 12
 
+    let x2 = await fn(4).then((a: int) => (discard)).then(() => 13)
+    doAssert x2 == 13
+
+  block: # catch
+    var reason: Error
+    await fn(6).then((a: int) => (witness.add $a)).catch((r: Error) => (reason = r))
+    doAssert reason == nil
+
+    await fn(7).then((a: int) => (discard)).catch((r: Error) => (reason = r))
+    doAssert reason != nil
+    doAssert reason.name == "Error"
+    doAssert "foobar: 7" in $reason.message
+  echo "done" # justified here to make sure we're running this, since it's inside `async`
+
+discard main()

tests/js/tasyncjs_fail.nim

@@ -0,0 +1,22 @@
+discard """
+  exitCode: 1
+ outputsub: "Error: unhandled exception: foobar: 13"
+"""
+
+# note: this needs `--unhandled-rejections=strict`, see D20210217T215950
+
+import std/asyncjs
+from std/sugar import `=>`
+
+proc fn(n: int): Future[int] {.async.} =
+  if n >= 7: raise newException(ValueError, "foobar: " & $n)
+  else: result = n
+
+proc main() {.async.} =
+  let x1 = await fn(6)
+  doAssert x1 == 6
+  await fn(7).catch((a: Error) => (discard))
+  let x3 = await fn(13)
+  doAssert false # shouldn't go here, should fail before
+
+discard main()