[JS/TS] Add F# quotation support (#4474)

Author: OnurGumus

src/Fable.Cli/CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 * [Python/Beam] Add F# quotation support — construction, pattern matching, and evaluation via `LeafExpressionConverter.EvaluateQuotation` (by @dbrattli)
+* [JS/TS] Add F# quotation support — construction, pattern matching, and evaluation (by @OnurGumus)
 * [All] Add support for `Guid.CreateVersion7()` and `Guid.CreateVersion7(DateTimeOffset)` (by @OnurGumus)
 * [All] Add missing `Array`, `List`, and `Seq` random choice/shuffle/sample members and tests (by @ncave)
 * [Dart/Rust] Add missing `System.Random` implementations and tests (by @ncave)

src/Fable.Compiler/CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 * [Python/Beam] Add F# quotation support — construction, pattern matching, and evaluation via `LeafExpressionConverter.EvaluateQuotation` (by @dbrattli)
+* [JS/TS] Add F# quotation support — construction, pattern matching, and evaluation (by @OnurGumus)
 * [All] Add support for `Guid.CreateVersion7()` and `Guid.CreateVersion7(DateTimeOffset)` (by @OnurGumus)
 * [All] Add missing `Array`, `List`, and `Seq` random choice/shuffle/sample members and tests (by @ncave)
 * [Dart/Rust] Add missing `System.Random` implementations and tests (by @ncave)

src/Fable.Core/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+* Add `Fable.Core.Quotations` namespace with `QuotExpr` and `QuotVar` types for .NET-side deserialization of Fable quotation ASTs
+
 ## 5.0.0-rc.1 - 2026-02-26
 
 ### Added

src/Fable.Core/Fable.Core.Quotations.fs

@@ -0,0 +1,36 @@
+namespace Fable.Core.Quotations
+
+/// Serializable variable in a quotation expression.
+/// Matches the JSON shape produced by Fable's quotation runtime.
+type QuotVar =
+    {
+        Name: string
+        Type: string
+        IsMutable: bool
+    }
+
+/// Serializable quotation expression tree.
+/// Designed for cross-platform JSON deserialization (e.g. via Thoth.Json)
+/// of quotations constructed on Fable JS/TS clients.
+///
+/// JSON format: ["Tag", ...fields] arrays, e.g. ["Value", 42, "int32"]
+[<RequireQualifiedAccess>]
+type QuotExpr =
+    | Value of value: obj * typeName: string
+    | Var of var: QuotVar
+    | Lambda of var: QuotVar * body: QuotExpr
+    | Application of func: QuotExpr * arg: QuotExpr
+    | Let of var: QuotVar * value: QuotExpr * body: QuotExpr
+    | IfThenElse of guard: QuotExpr * thenExpr: QuotExpr * elseExpr: QuotExpr
+    | Call of instance: QuotExpr option * methodName: string * args: QuotExpr array
+    | Sequential of first: QuotExpr * second: QuotExpr
+    | NewTuple of elements: QuotExpr array
+    | TupleGet of tuple: QuotExpr * index: int
+    | NewUnion of typeName: string * tag: int * fields: QuotExpr array
+    | UnionTag of expr: QuotExpr
+    | UnionField of expr: QuotExpr * index: int
+    | NewRecord of fieldNames: string array * values: QuotExpr array
+    | FieldGet of expr: QuotExpr * fieldName: string
+    | FieldSet of expr: QuotExpr * fieldName: string * value: QuotExpr
+    | VarSet of target: QuotExpr * value: QuotExpr
+    | NewList of head: QuotExpr * tail: QuotExpr

src/Fable.Core/Fable.Core.fsproj

@@ -21,6 +21,7 @@
     <Compile Include="Fable.Core.RustInterop.fs" />
     <Compile Include="Fable.Core.BeamInterop.fs" />
     <Compile Include="Fable.Core.Extensions.fs" />
+    <Compile Include="Fable.Core.Quotations.fs" />
     <Content Include="CHANGELOG.md" />
   </ItemGroup>
   <ItemGroup>

src/Fable.Transforms/Fable2Babel.fs

@@ -3142,7 +3142,9 @@ but thanks to the optimisation done below we get
             | Fable.Throw _
             | Fable.Debugger -> iife com ctx expr
 
-        | Fable.Quote _ -> addErrorAndReturnNull com None "Quotations are not yet supported for JS/TS target"
+        | Fable.Quote(quotedExpr, _isTyped, _r) ->
+            let emitted = QuotationEmitter.emitQuotedExpr com quotedExpr
+            transformAsExpr com ctx emitted
 
     let rec transformAsStatements (com: IBabelCompiler) ctx returnStrategy (expr: Fable.Expr) : Statement array =
         match expr with
@@ -3311,9 +3313,9 @@ but thanks to the optimisation done below we get
                 )
             |]
 
-        | Fable.Quote _ ->
-            addError com [] None "Quotations are not yet supported for JS/TS target"
-            [||]
+        | Fable.Quote(quotedExpr, _isTyped, _r) ->
+            let emitted = QuotationEmitter.emitQuotedExpr com quotedExpr
+            [| transformAsExpr com ctx emitted |> resolveExpr Fable.Any returnStrategy |]
 
     let transformFunction com ctx name (args: Fable.Ident list) (body: Fable.Expr) : Parameter array * BlockStatement =
         let tailcallChance =

src/Fable.Transforms/Replacements.fs

@@ -1309,6 +1309,8 @@ let operators (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Expr o
     // Erased operators.
     // KeyValuePair is already compiled as a tuple
     | ("KeyValuePattern" | "Identity" | "Box" | "Unbox" | "ToEnum"), [ arg ] -> TypeCast(arg, t) |> Some
+    // Quotation splice: %expr inside a quotation. The arg is already a QuotExpr at runtime.
+    | ("SpliceExpression" | "SpliceUntypedExpression"), [ arg ] -> Some arg
     // Cast to unit to make sure nothing is returned when wrapped in a lambda, see #1360
     | "Ignore", _ -> TypeCast(args.Head, Unit) |> Some
     // Number and String conversions
@@ -4344,7 +4346,8 @@ let tryCall (com: ICompiler) (ctx: Context) r t (info: CallInfo) (thisArg: Expr
                 getTypeName com ctx loc exprType |> StringConstant |> makeValue r |> Some
             | c -> Helper.LibCall(com, "Reflection", "name", t, [ c ], ?loc = r) |> Some
         | _ -> None
-    | _ -> None
+    // F# Quotations
+    | typeName -> Quotations.tryQuotationCall "quotation" com ctx r t info thisArg args typeName
 
 let tryBaseConstructor com ctx (ent: EntityRef) (argTypes: Lazy<Type list>) genArgs args =
     match ent.FullName with

src/fable-library-ts/Fable.Library.TypeScript.fsproj

@@ -65,6 +65,7 @@
     <TypeScriptCompile Include="Timer.ts" />
     <TypeScriptCompile Include="TimeSpan.ts" />
     <Content Include="tsconfig.json" />
+    <TypeScriptCompile Include="quotation.ts" />
     <TypeScriptCompile Include="Types.ts" />
     <TypeScriptCompile Include="Unicode.13.0.0.ts" />
     <TypeScriptCompile Include="Uri.ts" />