Merge pull request #161 from Rich-Harris/gh-159

preserve parens around x in y in for loop head

src/program/types/ParenthesizedExpression.js

@@ -21,6 +21,16 @@ function shouldRemoveParens ( expression, parent ) {
 		if ( parent.left.contains( expression ) ) return false;
 	}
 
+	// special case — `for (x=(y in z);...)`. This is a bit ugly
+	if ( expression.type === 'BinaryExpression' && expression.operator === 'in' ) {
+		let maybeForStatement = parent;
+		while ( maybeForStatement && maybeForStatement.type !== 'ForStatement' ) {
+			maybeForStatement = maybeForStatement.parent;
+		}
+
+		if ( maybeForStatement && maybeForStatement.init && maybeForStatement.init.contains( parent ) ) return false;
+	}
+
 	const expressionPrecedence = expression.getPrecedence();
 	const parentPrecedence = parent.getPrecedence();
 

test/samples/loops.js

@@ -245,5 +245,23 @@ module.exports = [
 				do_anything()
 			}`,
 		output: `for(let f=1;condition;f++)do_anything()`
+	},
+
+	{
+		description: 'preserves parens around `x in y` expression',
+		input: `
+			for (any_value = ("key" in any_object); any_condition;) {
+				any_fn()
+			}`,
+		output: `for(any_value=("key" in any_object);any_condition;)any_fn()`
+	},
+
+	{
+		description: 'preserves parens around `x in y` expression in var declaration',
+		input: `
+			for (var any_value = ("key" in any_object); any_condition;) {
+				any_fn()
+			}`,
+		output: `for(var any_value=("key" in any_object);any_condition;)any_fn()`
 	}
 ];