Fix destructuring with holes in assign pattern (#14240)

packages/babel-plugin-transform-destructuring/src/index.ts

@@ -114,8 +114,16 @@ export default declare((api, options) => {
           ),
         );
       } else {
+        let nodeInit: t.Expression;
+
+        if (this.kind === "const" && init === null) {
+          nodeInit = this.scope.buildUndefinedNode();
+        } else {
+          nodeInit = t.cloneNode(init);
+        }
+
         node = t.variableDeclaration(this.kind, [
-          t.variableDeclarator(id, t.cloneNode(init)),
+          t.variableDeclarator(id, nodeInit),
         ]);
       }
 
@@ -158,6 +166,14 @@ export default declare((api, options) => {
     }
 
     pushAssignmentPattern({ left, right }, valueRef) {
+      // handle array init hole
+      // const [x = 42] = [,];
+      // -> const x = 42;
+      if (valueRef === null) {
+        this.nodes.push(this.buildVariableAssignment(left, right));
+        return;
+      }
+
       // we need to assign the current value of the assignment to avoid evaluating
       // it more than once
       const tempId = this.scope.generateUidIdentifierBasedOnNode(valueRef);

packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/init-hole/exec.js

@@ -0,0 +1,45 @@
+let [x = 23] = [,];
+expect(x).toEqual(23);
+
+const [y = 24, z] = [, 42];
+expect(y).toEqual(24);
+expect(z).toEqual(42);
+
+function* foo() {
+    yield 1;
+    yield 2;
+}
+
+let bar = foo();
+
+const [a = bar.next().value, b] = [, bar.next().value];
+
+expect(a).toEqual(2);
+expect(b).toEqual(1);
+
+const arr = [c = 42] = [,];
+expect(c).toEqual(42);
+expect(arr).toEqual([,]);
+
+var iterCount = 0;
+
+for (const [x = 23] = [,]; iterCount < 1; ) {
+    expect(x).toEqual(23);
+    // another statement
+
+    iterCount += 1;
+}
+
+expect(iterCount).toEqual(1);
+
+const [...d] = [,];
+const [...{ 0: e }] = [,];
+
+expect(d).toEqual([,]);
+expect(e).toEqual(undefined);
+
+const [f] = [,];
+expect(f).toEqual(undefined);
+
+let [g] = [,];
+expect(g).toEqual(undefined);

packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/init-hole/input.js

@@ -0,0 +1,45 @@
+let [x = 23] = [,];
+expect(x).toEqual(23);
+
+const [y = 24, z] = [, 42];
+expect(y).toEqual(24);
+expect(z).toEqual(42);
+
+function* foo() {
+    yield 1;
+    yield 2;
+}
+
+let bar = foo();
+
+const [a = bar.next().value, b] = [, bar.next().value];
+
+expect(a).toEqual(2);
+expect(b).toEqual(1);
+
+const arr = [c = 42] = [,];
+expect(c).toEqual(42);
+expect(arr).toEqual([,]);
+
+var iterCount = 0;
+
+for (const [x = 23] = [,]; iterCount < 1; ) {
+    expect(x).toEqual(23);
+    // another statement
+
+    iterCount += 1;
+}
+
+expect(iterCount).toEqual(1);
+
+const [...d] = [,];
+const [...{ 0: e }] = [,];
+
+expect(d).toEqual([,]);
+expect(e).toEqual(undefined);
+
+const [f] = [,];
+expect(f).toEqual(undefined);
+
+let [g] = [,];
+expect(g).toEqual(undefined);

packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/init-hole/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["transform-destructuring"]
+}

packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/init-hole/output.js

@@ -0,0 +1,41 @@
+var _ref2, _ref2$;
+
+let x = 23;
+expect(x).toEqual(23);
+const y = 24,
+      z = 42;
+expect(y).toEqual(24);
+expect(z).toEqual(42);
+
+function* foo() {
+  yield 1;
+  yield 2;
+}
+
+let bar = foo();
+const _ref = [, bar.next().value],
+      _ref$ = _ref[0],
+      a = _ref$ === void 0 ? bar.next().value : _ref$,
+      b = _ref[1];
+expect(a).toEqual(2);
+expect(b).toEqual(1);
+const arr = (_ref2 = [,], _ref2$ = _ref2[0], c = _ref2$ === void 0 ? 42 : _ref2$, _ref2);
+expect(c).toEqual(42);
+expect(arr).toEqual([,]);
+var iterCount = 0;
+
+for (const x = 23; iterCount < 1;) {
+  expect(x).toEqual(23); // another statement
+
+  iterCount += 1;
+}
+
+expect(iterCount).toEqual(1);
+const d = [,];
+const e = [,][0];
+expect(d).toEqual([,]);
+expect(e).toEqual(undefined);
+const f = void 0;
+expect(f).toEqual(undefined);
+let g;
+expect(g).toEqual(undefined);