Fix #951: properly parenthesize ** expressions (#952)

src/fast-path.js

@@ -232,19 +232,6 @@ FPp.needsParens = function(assumeExpressionContext) {
     return true;
   }
 
-  // The left-hand side of the ** exponentiation operator must always
-  // be parenthesized unless it's an ident or literal
-  if (
-    parent.type === "BinaryExpression" &&
-    parent.operator === "**" &&
-    parent.left === node &&
-    node.type !== "Identifier" &&
-    node.type !== "Literal" &&
-    node.type !== "NumericLiteral"
-  ) {
-    return true;
-  }
-
   if (
     parent.type === "ArrowFunctionExpression" && parent.body === node && startsWithNoLookaheadToken(node, /* forbidFunctionAndClass */ false)
     || parent.type === "ExpressionStatement" && startsWithNoLookaheadToken(node, /* forbidFunctionAndClass */ true)
@@ -285,6 +272,9 @@ FPp.needsParens = function(assumeExpressionContext) {
         case "CallExpression":
           return name === "callee" && parent.callee == node;
 
+        case "BinaryExpression":
+          return parent.operator === "**" && name === "left";
+
         default:
           return false;
       }
@@ -332,6 +322,10 @@ FPp.needsParens = function(assumeExpressionContext) {
             return true;
           }
 
+          if (no === "**" && po === "**") {
+            return name === "left";
+          }
+
           if (pp === np && name === "right") {
             assert.strictEqual(parent.right, node);
             return true;

src/printer.js

@@ -2783,10 +2783,12 @@ function printBinaryishExpressions(path, parts, print, options, isNested) {
     // is where the rest of the expression will exist. Binary
     // expressions on the right side mean they have a difference
     // precedence level and should be treated as a separate group, so
-    // print them normally.
+    // print them normally. (This doesn't hold for the `**` operator,
+    // which is unique in that it is right-associative.)
     if (
       util.getPrecedence(node.left.operator) ===
       util.getPrecedence(node.operator)
+      && node.operator !== "**"
     ) {
       // Flatten them out by recursively calling this function. The
       // printed values will all be appended to `parts`.

src/util.js

@@ -294,7 +294,8 @@ var PRECEDENCE = {};
   ["<", ">", "<=", ">=", "in", "instanceof"],
   [">>", "<<", ">>>"],
   ["+", "-"],
-  ["*", "/", "%", "**"]
+  ["*", "/", "%"],
+  ["**"]
 ].forEach(function(tier, i) {
   tier.forEach(function(op) {
     PRECEDENCE[op] = i;

tests/binary-expressions/__snapshots__/jsfmt.spec.js.snap

@@ -19,6 +19,27 @@ function f() {
 "
 `;
 
+exports[`exp.js 1`] = `
+"a ** b ** c;
+(a ** b) ** c;
+a.b ** c;
+(-a) ** b;
+a ** -b;
+-(a**b);
+(a * b) ** c;
+a ** (b * c);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+a ** b ** c;
+(a ** b) ** c;
+a.b ** c;
+(-a) ** b;
+a ** -b;
+-(a ** b);
+(a * b) ** c;
+a ** (b * c);
+"
+`;
+
 exports[`inline-object-array.js 1`] = `
 "prevState = prevState || {
   catalogs: [],

tests/binary-expressions/exp.js

@@ -0,0 +1,8 @@
+a ** b ** c;
+(a ** b) ** c;
+a.b ** c;
+(-a) ** b;
+a ** -b;
+-(a**b);
+(a * b) ** c;
+a ** (b * c);