Fable Plugin to keep integer arithmetic in JavaScript

Fable.Plugins.BitwiseWrap.fsx

namespace Fable.Plugins

#r "../../build/fable/bin/Fable.exe"

open Fable.AST
open Fable.AST.Fable
open Fable.FSharp2Fable

type RandomPlugin() =
    interface IReplacePlugin with
        member x.TryReplace com (info: Fable.ApplyInfo) =

            match info.ownerFullName with
            | "Microsoft.FSharp.Core.ExtraTopLevelOperators"
            | "Microsoft.FSharp.Core.Operators" ->
                let patternFor t = 
                    match t with
                    | PrimitiveType (Number kind) -> 
                        match kind with
                        | Int8 -> Some "($0 + 0x80 & 0xFF) - 0x80"
                        | UInt8 -> Some "$0 & 0xFF"
                        | Int16 -> Some "($0 + 0x8000 & 0xFFFF) - 0x8000"
                        | UInt16 -> Some "$0 & 0xFFFF"
                        | Int32 -> Some "($0 + 0x80000000 >>> 0) - 0x80000000"
                        | UInt32 -> Some "$0 >>> 0"
                        | _ -> None
                    | _ -> None

                let applyMask t args =
                    match patternFor t with
                    | Some pattern ->
                        pattern
                        |> Fable.Replacements.Util.emit info <| args
                        |> Some
                    | _ -> None

                match info.methodName with
                | "sbyte"
                | "byte"
                | "int8"
                | "uint8"
                | "int16"
                | "uint16"
                | "int"
                | "int32"
                | "uint"
                | "uint32"
                | "~~~"
                | "~-" 
                | "~+" ->
                    if info.args.Length <> 1 then
                        failwithf "Unexpected arg count for '%s'" info.methodName

                    applyMask info.returnType info.args

                | "+" 
                | "-" 
                | "*" 
                | "/" 
                | "%"
                | "<<<" 
                | ">>>" 
                | "&&&" 
                | "|||" 
                | "^^^" ->
                    if info.args.Length <> 2 then
                        failwithf "Unexpected arg count for %s" info.methodName

                    match Fable.Replacements.Util.applyOp com info info.args info.methodName with
                    | Some expr -> applyMask info.returnType [expr]
                    | _ -> None

                | _ -> None

            | _ -> None
            

Great tips! Working on it.

Also, I did somehow manage to completely miss the part you said about Babel managing the precedence order of operators. I see now you clearly state it in docs/plugins.md.

I know... I know... "More sleep, less coffee!" :)

Second version. Now with Alfonso's suggestions and handling signed types.

Third version. Fix a problem with signed int32.

& 0xFFFFFFFF preserves the signal (hence, does nothing :) ). Now uses >>> 0 to convert to unsigned.

Source: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators

Fourth version. Adds support to Microsoft.FSharp.Core.ExtraTopLevelOperators.int8/uint8.