From 99a26d50f510c83f10c266db2edc000a06d5acf5 Mon Sep 17 00:00:00 2001
From: Preston Taylor <95388976+PrestonLTaylor@users.noreply.github.com>
Date: Thu, 18 Apr 2024 15:45:49 +0100
Subject: [PATCH] feat: Implement execution of function expressions

---
 JSS.Lib.UnitTests/ASTTests.cs     | 14 ++++++
 JSS.Lib/AST/FunctionExpression.cs | 80 ++++++++++++++++++++++++++++++-
 2 files changed, 92 insertions(+), 2 deletions(-)

diff --git a/JSS.Lib.UnitTests/ASTTests.cs b/JSS.Lib.UnitTests/ASTTests.cs
index 9e60a3b..a5658b5 100644
--- a/JSS.Lib.UnitTests/ASTTests.cs
+++ b/JSS.Lib.UnitTests/ASTTests.cs
@@ -1310,6 +1310,20 @@ public void ConstDeclarations_InsideFunctionDeclaration_DontEscapeToOuterScope()
         completion.Value.Should().Be(new String($"{identifier} is not defined"));
     }
 
+    [Test]
+    public void AssignmentExpreesion_ToFunctionExpression_SetsIdentifier_ToFunction()
+    {
+        // Arrange
+        var script = ParseScript("a = function() { return 1 }; a()");
+
+        // Act
+        var completion = script.ScriptEvaluation();
+
+        // Assert
+        completion.IsNormalCompletion().Should().BeTrue();
+        completion.Value.Should().Be(new Number(1));
+    }
+
     static private string EscapeString(string toEscape, char quote = '"')
     {
         return $"{quote}{toEscape}{quote}";
diff --git a/JSS.Lib/AST/FunctionExpression.cs b/JSS.Lib/AST/FunctionExpression.cs
index b455d6c..777ba7b 100644
--- a/JSS.Lib/AST/FunctionExpression.cs
+++ b/JSS.Lib/AST/FunctionExpression.cs
@@ -1,4 +1,7 @@
-namespace JSS.Lib.AST;
+using JSS.Lib.AST.Values;
+using JSS.Lib.Execution;
+
+namespace JSS.Lib.AST;
 
 // 15.2 Function Definitions, https://tc39.es/ecma262/#sec-function-definitions
 internal sealed class FunctionExpression : IExpression
@@ -10,7 +13,80 @@ public FunctionExpression(string? identifier, List<Identifier> parameters, State
         Body = body;
     }
 
-    // FIXME: 15.2.6 Runtime Semantics: Evaluation, https://tc39.es/ecma262/#sec-function-definitions-runtime-semantics-evaluation
+    // 15.2.6 Runtime Semantics: Evaluation, https://tc39.es/ecma262/#sec-function-definitions-runtime-semantics-evaluation
+    public override Completion Evaluate(VM vm)
+    {
+        // 1. Return InstantiateOrdinaryFunctionExpression of FunctionExpression.
+        if (Identifier is null)
+        {
+            return InstantiateOrdinaryFunctionExpressionWithNoName(vm);
+        }
+        else
+        {
+            return InstantiateOrdinaryFunctionExpression(vm);
+        }
+    }
+
+    // 15.2.5 Runtime Semantics: InstantiateOrdinaryFunctionExpression, https://tc39.es/ecma262/#sec-runtime-semantics-instantiateordinaryfunctionexpression
+    private Completion InstantiateOrdinaryFunctionExpressionWithNoName(VM vm, string? name = null)
+    {
+        // 1. If name is not present, set name to "".
+        name ??= "";
+
+        // 2. Let env be the LexicalEnvironment of the running execution context.
+        var env = (vm.CurrentExecutionContext as ScriptExecutionContext)!.LexicalEnvironment;
+
+        // FIXME: 3. Let privateEnv be the running execution context's PrivateEnvironment.
+
+        // FIXME: 4. Let sourceText be the source text matched by FunctionExpression.
+
+        // 5. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, NON-LEXICAL-THIS, env, privateEnv).
+        var closure = FunctionObject.OrdinaryFunctionCreate(Parameters, Body, LexicalThisMode.NON_LEXICAL_THIS, env!);
+
+        // 6. Perform SetFunctionName(closure, name).
+        closure.SetFunctionName(name);
+
+        // 7. Perform MakeConstructor(closure).
+        closure.MakeConstructor();
+
+        // 8. Return closure.
+        return closure;
+    }
+
+    private Completion InstantiateOrdinaryFunctionExpression(VM vm)
+    {
+        // 1. Assert: name is not present.
+        // 2. Set name to StringValue of BindingIdentifier.
+        var name = Identifier!;
+
+        // 3. Let outerEnv be the running execution context's LexicalEnvironment.
+        var outerEnv = (vm.CurrentExecutionContext as ScriptExecutionContext)!.LexicalEnvironment;
+
+        // 4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
+        var funcEnv = new DeclarativeEnvironment(outerEnv);
+
+        // 5. Perform ! funcEnv.CreateImmutableBinding(name, false).
+        MUST(funcEnv.CreateImmutableBinding(name, false));
+
+        // FIXME: 6. Let privateEnv be the running execution context's PrivateEnvironment.
+
+        // FIXME: 7. Let sourceText be the source text matched by FunctionExpression.
+
+        // 8. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, NON-LEXICAL-THIS, funcEnv, privateEnv).
+        var closure = FunctionObject.OrdinaryFunctionCreate(Parameters, Body, LexicalThisMode.NON_LEXICAL_THIS, funcEnv);
+
+        // 9. Perform SetFunctionName(closure, name).
+        closure.SetFunctionName(name);
+
+        // 10. Perform MakeConstructor(closure).
+        closure.MakeConstructor();
+
+        // 11. Perform ! funcEnv.InitializeBinding(name, closure).
+        MUST(funcEnv.InitializeBinding(name, closure));
+
+        // 12. Return closure.
+        return closure;
+    }
 
     public string? Identifier { get; }
     public IReadOnlyList<Identifier> Parameters { get; }