Array.prototype.join

Trivial

compiler/forget/src/HIR/ObjectShape.ts

@@ -230,6 +230,16 @@ addObject(BUILTIN_SHAPES, BuiltInArrayId, [
       returnValueKind: ValueKind.Mutable,
     }),
   ],
+  [
+    "join",
+    addFunction(BUILTIN_SHAPES, [], {
+      positionalParams: [],
+      restParam: Effect.ConditionallyMutate,
+      returnType: PRIMITIVE_TYPE,
+      calleeEffect: Effect.Read,
+      returnValueKind: ValueKind.Immutable,
+    }),
+  ],
   // TODO: rest of Array properties
 ]);
 

compiler/forget/src/__tests__/fixtures/compiler/array-join.expect.md

@@ -0,0 +1,68 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  const x = [{}, [], props.value];
+  const y = x.join(() => "this closure gets stringified, not called");
+  foo(y);
+  return [x, y];
+}
+
+```
+
+## Code
+
+```javascript
+import { unstable_useMemoCache as useMemoCache } from "react";
+function Component(props) {
+  const $ = useMemoCache(8);
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = {};
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  let t1;
+  if ($[1] === Symbol.for("react.memo_cache_sentinel")) {
+    t1 = [];
+    $[1] = t1;
+  } else {
+    t1 = $[1];
+  }
+  const c_2 = $[2] !== props.value;
+  let t2;
+  if (c_2) {
+    t2 = [t0, t1, props.value];
+    $[2] = props.value;
+    $[3] = t2;
+  } else {
+    t2 = $[3];
+  }
+  const x = t2;
+  let t3;
+  if ($[4] === Symbol.for("react.memo_cache_sentinel")) {
+    t3 = () => "this closure gets stringified, not called";
+    $[4] = t3;
+  } else {
+    t3 = $[4];
+  }
+  const y = x.join(t3);
+  foo(y);
+  const c_5 = $[5] !== x;
+  const c_6 = $[6] !== y;
+  let t4;
+  if (c_5 || c_6) {
+    t4 = [x, y];
+    $[5] = x;
+    $[6] = y;
+    $[7] = t4;
+  } else {
+    t4 = $[7];
+  }
+  return t4;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/compiler/array-join.js

@@ -0,0 +1,6 @@
+function Component(props) {
+  const x = [{}, [], props.value];
+  const y = x.join(() => "this closure gets stringified, not called");
+  foo(y);
+  return [x, y];
+}