implement PrepareForTailCall

src/abstract-ops/function-operations.mjs

@@ -400,9 +400,11 @@ export function CreateBuiltinFunction(steps, internalSlotsList, realm, prototype
 
 // 14.9.3 #sec-preparefortailcall
 export function PrepareForTailCall() {
-  // const leafContext = surroundingAgent.runningExecutionContext;
-  // Suspend(leafContext);
-  // surroundingAgent.executionContextStack.pop();
-  // Assert: leafContext has no further use. It will never
-  // be activated as the running execution context.
+  // 1. Let leafContext be the running execution context.
+  const leafContext = surroundingAgent.runningExecutionContext;
+  // 2. Suspend leafContext.
+  // 3. Pop leafContext from the execution context stack. The execution context now on the top of the stack becomes the running execution context.
+  surroundingAgent.executionContextStack.pop(leafContext);
+  // 4. Assert: leafContext has no further use. It will never be activated as the running execution context.
+  leafContext.poppedForTailCall = true;
 }

src/engine.mjs

@@ -52,10 +52,10 @@ export class Agent {
 
     this.executionContextStack = [];
     const stackPop = this.executionContextStack.pop;
-    this.executionContextStack.pop = function pop(...args) {
-      const popped = stackPop.call(this);
-      if (args.length === 1) {
-        Assert(args[0] === popped);
+    this.executionContextStack.pop = function pop(ctx) {
+      if (!ctx.poppedForTailCall) {
+        const popped = stackPop.call(this);
+        Assert(popped === ctx);
       }
     };
 
@@ -159,6 +159,7 @@ export class ExecutionContext {
     // NON-SPEC
     this.callSite = new CallSite(this);
     this.promiseCapability = undefined;
+    this.poppedForTailCall = false;
   }
 
   copy() {