convert Error to System.Exception

src/bridge.fs

@@ -132,6 +132,7 @@ module internal Bridge =
         |> fixNodeArray
         |> fixReadonlyArray
         |> fixDateTime
+        |> mapErrorToException
         |> fixStatic
         |> createIExports
         |> fixOverloadingOnStringParameters // fixEscapeWords must be after

src/transform.fs

@@ -403,6 +403,15 @@ let createIExports (f: FsFile): FsFile =
             )
     }
 
+let flattenInherits (i:FsInterface) : FsType seq = seq {
+    for b in i.Inherits do
+        yield b
+        match b with
+        | FsType.Interface i2 ->
+            yield! flattenInherits i2
+        | _ -> ()
+}
+
 let fixTic (typeParameters: FsType list) (tp: FsType) =
     if typeParameters.Length = 0 then
         tp
@@ -525,6 +534,25 @@ let fixDateTime(f: FsFile): FsFile =
         | _ -> tp
     )
 
+/// https://github.com/fable-compiler/ts2fable/issues/298#issuecomment-478328229
+let mapErrorToException (f: FsFile): FsFile =
+    let simpleExceptionType = simpleType "System.Exception"
+    f |> fixFile (fun tp ->
+        match tp with
+
+        // if the type IS error, map it to System.Exception
+        | FsType.Mapped m when (m.Name = "Error") ->
+            simpleExceptionType
+
+        // if the type INHERITS from error, replace it with System.Exception.
+        // This loses information, but at least compiles.
+        // see https://github.com/fable-compiler/ts2fable/issues/298#issuecomment-504104618
+        | FsType.Interface i when (flattenInherits i |> Seq.contains simpleExceptionType) ->
+            FsType.Alias { Name = i.Name; Type = simpleExceptionType; TypeParameters = [] }
+
+        | _ -> tp
+    )
+
 let fixEnumReferences (f: FsFile): FsFile =
     // get a list of enum names
     let list = List<string>()

test/fragments/regressions/#xxx-error-to-exception.d.ts

@@ -0,0 +1,13 @@
+
+export module ErrorExceptionTest {
+    // should be System.Exception
+    var instanceErrorProperty: Error;
+
+    // should be replaced with an alias to System.Exception, "i" will not be mapped.
+    interface InheritFromError extends Error {
+        i : number
+    }
+
+    var instanceInheritFromErrorProperty: InheritFromError;
+}
+

test/fragments/regressions/#xxx-error-to-exception.expected.fs

@@ -0,0 +1,16 @@
+// ts2fable 0.0.0
+module rec #xxx-error-to-exception
+open System
+open Fable.Core
+open Fable.Import.JS
+
+let [<Import("ErrorExceptionTest","test")>] errorExceptionTest: ErrorExceptionTest.IExports = jsNative
+
+module ErrorExceptionTest =
+
+    type [<AllowNullLiteral>] IExports =
+        abstract instanceErrorProperty: System.Exception
+        abstract instanceInheritFromErrorProperty: InheritFromError
+
+    type InheritFromError =
+        System.Exception

test/fsFileTests.fs

@@ -381,3 +381,7 @@ describe "transform tests" <| fun _ ->
     // https://github.com/fable-compiler/ts2fable/issues/292
     it "regression #292 static props" <| fun _ ->
         runRegressionTest "#292-static-props"
+
+    // https://github.com/fable-compiler/ts2fable/issues/xxx
+    it "regression #xxx error to exception" <| fun _ ->
+        runRegressionTest "#xxx-error-to-exception"