[JS] Fix casting to ints (#22327)

* [JS] Fix casting to ints

* Simplify `genCast` by using `asUintN`/`asIntN`

2 files changed, 28 insertions, 19 deletions

diff --git a/compiler/jsgen.nim b/compiler/jsgen.nim
index 1fbf6c74c..4a62cbf9e 100644
--- a/compiler/jsgen.nim
+++ b/compiler/jsgen.nim
@@ -2727,26 +2727,14 @@ proc genCast(p: PProc, n: PNode, r: var TCompRes) =
   let fromInt = (src.kind in tyInt..tyInt32)
   let fromUint = (src.kind in tyUInt..tyUInt32)
 
-  if toUint and (fromInt or fromUint):
-    let trimmer = unsignedTrimmer(dest.size)
-    r.res = "($1 $2)" % [r.res, trimmer]
-  elif toUint and src.kind in {tyInt64, tyUInt64} and optJsBigInt64 in p.config.globalOptions:
-    r.res = "Number(BigInt.asUintN($1, $2))" % [$(dest.size * 8), r.res]
+  if toUint:
+    if fromInt or fromUint:
+      r.res = "Number(BigInt.asUintN($1, BigInt($2)))" % [$(dest.size * 8), r.res]
+    elif src.kind in {tyInt64, tyUInt64} and optJsBigInt64 in p.config.globalOptions:
+      r.res = "Number(BigInt.asUintN($1, $2))" % [$(dest.size * 8), r.res]
   elif toInt:
-    if fromInt:
-      return
-    elif fromUint:
-      if src.size == 4 and dest.size == 4:
-        # XXX prevent multi evaluations
-        r.res = "($1 | 0)" % [r.res]
-      else:
-        let trimmer = unsignedTrimmer(dest.size)
-        let minuend = case dest.size
-          of 1: "0xfe"
-          of 2: "0xfffe"
-          of 4: "0xfffffffe"
-          else: ""
-        r.res = "($1 - ($2 $3))" % [rope minuend, r.res, trimmer]
+    if fromInt or fromUint:
+      r.res = "Number(BigInt.asIntN($1, BigInt($2)))" % [$(dest.size * 8), r.res]
     elif src.kind in {tyInt64, tyUInt64} and optJsBigInt64 in p.config.globalOptions:
       r.res = "Number(BigInt.asIntN($1, $2))" % [$(dest.size * 8), r.res]
   elif dest.kind == tyInt64 and optJsBigInt64 in p.config.globalOptions:
diff --git a/tests/cast/tcast.nim b/tests/cast/tcast.nim
new file mode 100644
index 000000000..205444ea3
--- /dev/null
+++ b/tests/cast/tcast.nim
@@ -0,0 +1,21 @@
+discard """
+  targets: "c cpp js"
+"""
+
+proc main() =
+  block: # bug #16806
+    let
+      a = 42u16
+      b = cast[int16](a)
+    doAssert a.int16 == 42
+    doAssert b in int16.low..int16.high
+
+  block: # bug #16808
+    doAssert cast[int8](cast[uint8](int8(-12))) == int8(-12)
+    doAssert cast[int16](cast[uint16](int16(-12))) == int16(-12)
+    doAssert cast[int32](cast[uint32](int32(-12))) == int32(-12)
+
+  doAssert cast[int8](int16.high) == -1
+
+static: main()
+main()