Fix parenthesis for nullish coalescing

packages/babel-generator/src/generators/expressions.js

@@ -227,10 +227,8 @@ export function BindExpression(node: Object) {
   this.print(node.callee, node);
 }
 
-export {
-  AssignmentExpression as BinaryExpression,
-  AssignmentExpression as LogicalExpression,
-};
+export { AssignmentExpression as BinaryExpression };
+export { AssignmentExpression as LogicalExpression };
 
 export function MemberExpression(node: Object) {
   this.print(node.object, node);

packages/babel-generator/src/node/index.js

@@ -96,6 +96,18 @@ export function needsParens(node, parent, printStack) {
   if (t.isNewExpression(parent) && parent.callee === node) {
     if (isOrHasCallExpression(node)) return true;
   }
+  /* this check if for NullishCoalescing with LogicalOperators liks && and ||
+   * For example when someone creates an ast programmaticaly like this
+   * t.logicalExpression(
+   * "??",
+   *  t.logicalExpression("||", t.identifier("a"), t.identifier("b")),
+   *  t.identifier("c"),
+   *  );
+   * In the above case we should that AST is equivalent to writing a || b ?? c
+   * This is incorrect because NullishCoalescing when used with LogicalExpressions should have parenthesis
+   * The correct syntax is (a || b) ?? c, that is why we need parens in this case
+   */
+  if (t.isLogicalExpression(node) && parent.operator === "??") return true;
 
   return find(expandedParens, node, parent, printStack);
 }

packages/babel-generator/test/fixtures/types/LogicalExpression/input.js

@@ -1,4 +1,5 @@
 foo ||bar;
 (x => x)|| bar;
 (function a(x){return x;})|| 2;
-0||(function(){return alpha;});
+0||(function(){return alpha;});
+a ?? (b || c);

packages/babel-generator/test/fixtures/types/LogicalExpression/options.json

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

packages/babel-generator/test/fixtures/types/LogicalExpression/output.js

@@ -7,4 +7,6 @@ foo || bar;
 
 0 || function () {
   return alpha;
-};
+};
+
+a ?? (b || c);

packages/babel-generator/test/index.js

@@ -298,6 +298,29 @@ describe("generation", function() {
 });
 
 describe("programmatic generation", function() {
+  it("should add parenthesis when NullishCoalescing is used along with ||", function() {
+    // https://github.com/babel/babel/issues/10260
+    const nullishCoalesc = t.logicalExpression(
+      "??",
+      t.logicalExpression("||", t.identifier("a"), t.identifier("b")),
+      t.identifier("c"),
+    );
+    const output = generate(nullishCoalesc).code;
+    expect(output).toBe(`(a || b) ?? c`);
+  });
+  it("should add parenthesis when NullishCoalesing is used with &&", function() {
+    const nullishCoalesc = t.logicalExpression(
+      "??",
+      t.identifier("a"),
+      t.logicalExpression(
+        "&&",
+        t.identifier("b"),
+        t.logicalExpression("&&", t.identifier("c"), t.identifier("d")),
+      ),
+    );
+    const output = generate(nullishCoalesc).code;
+    expect(output).toBe(`a ?? (b && c && d)`);
+  });
   it("numeric member expression", function() {
     // Should not generate `0.foo`
     const mem = t.memberExpression(

packages/babel-parser/src/tokenizer/types.js

@@ -188,8 +188,8 @@ export const types: { [name: string]: TokenType } = {
   _null: createKeyword("null", { startsExpr }),
   _true: createKeyword("true", { startsExpr }),
   _false: createKeyword("false", { startsExpr }),
-  _in: createKeyword("in", { beforeExpr, binop: 7 }),
-  _instanceof: createKeyword("instanceof", { beforeExpr, binop: 7 }),
+  _in: createKeyword("in", { beforeExpr, binop: 8 }),
+  _instanceof: createKeyword("instanceof", { beforeExpr, binop: 8 }),
   _typeof: createKeyword("typeof", { beforeExpr, prefix, startsExpr }),
   _void: createKeyword("void", { beforeExpr, prefix, startsExpr }),
   _delete: createKeyword("delete", { beforeExpr, prefix, startsExpr }),