feat: add right-associative support for binary operators

fixes #195

.eslintrc.json

@@ -11,6 +11,14 @@
 		"browser": true,
 		"node": true
 	},
+	"overrides": [
+		{
+			"files": ["test/**/*.js", "*.test.js"],
+			"parserOptions": {
+				"ecmaVersion": 7
+			}
+		}
+	],
 	"rules": {
 		"semi": 1,
 		"no-dupe-args": 1,

README.md

@@ -61,8 +61,12 @@ const parse_tree = Jsep.parse('1 + 1');
 
 ```javascript
 // Add a custom ^ binary operator with precedence 10
+// (Note that higher number = higher precedence)
 jsep.addBinaryOp("^", 10);
 
+// Add exponentiation operator (right-to-left)
+jsep.addBinaryOp('**', 11, true);
+
 // Add a custom @ unary operator
 jsep.addUnaryOp('@');
 

src/jsep.js

@@ -36,11 +36,18 @@ export class Jsep {
 	 * @method jsep.addBinaryOp
 	 * @param {string} op_name The name of the binary op to add
 	 * @param {number} precedence The precedence of the binary op (can be a float). Higher number = higher precedence
+	 * @param {boolean} [isRightAssociative=false] whether operator is right-associative
 	 * @returns {Jsep}
 	 */
-	static addBinaryOp(op_name, precedence) {
+	static addBinaryOp(op_name, precedence, isRightAssociative) {
 		Jsep.max_binop_len = Math.max(op_name.length, Jsep.max_binop_len);
 		Jsep.binary_ops[op_name] = precedence;
+		if (isRightAssociative) {
+			Jsep.right_associative.add(op_name);
+		}
+		else {
+			Jsep.right_associative.delete(op_name);
+		}
 		return Jsep;
 	}
 
@@ -110,6 +117,7 @@ export class Jsep {
 		if (op_name.length === Jsep.max_binop_len) {
 			Jsep.max_binop_len = Jsep.getMaxKeyLen(Jsep.binary_ops);
 		}
+		Jsep.right_associative.delete(op_name);
 
 		return Jsep;
 	}
@@ -402,7 +410,7 @@ export class Jsep {
 
 		// Otherwise, we need to start a stack to properly place the binary operations in their
 		// precedence structure
-		biop_info = { value: biop, prec: Jsep.binaryPrecedence(biop)};
+		biop_info = { value: biop, prec: Jsep.binaryPrecedence(biop), right_a: Jsep.right_associative.has(biop) };
 
 		right = this.gobbleToken();
 
@@ -421,12 +429,15 @@ export class Jsep {
 				break;
 			}
 
-			biop_info = { value: biop, prec };
+			biop_info = { value: biop, prec, right_a: Jsep.right_associative.has(biop) };
 
 			cur_biop = biop;
 
 			// Reduce: make a binary expression from the three topmost entries.
-			while ((stack.length > 2) && (prec <= stack[stack.length - 2].prec)) {
+			const comparePrev = prev => biop_info.right_a && prev.right_a
+				? prec > prev.prec
+				: prec <= prev.prec;
+			while ((stack.length > 2) && comparePrev(stack[stack.length - 2])) {
 				right = stack.pop();
 				biop = stack.pop().value;
 				left = stack.pop();
@@ -929,6 +940,9 @@ Object.assign(Jsep, {
 		'*': 10, '/': 10, '%': 10
 	},
 
+	// sets specific binary_ops as right-associative
+	right_associative: new Set(),
+
 	// Additional valid identifier chars, apart from a-z, A-Z and 0-9 (except on the starting char)
 	additional_identifier_chars: new Set(['$', '_']),
 

test/jsep.test.js

@@ -61,15 +61,27 @@ import {testParser, testOpExpression, esprimaComparisonTest, resetJsepDefaults}
 		}, assert);
 	});
 
-	QUnit.test('Ops', function (assert) {
-		testOpExpression('1', assert);
-		testOpExpression('1+2', assert);
-		testOpExpression('1*2', assert);
-		testOpExpression('1*(2+3)', assert);
-		testOpExpression('(1+2)*3', assert);
-		testOpExpression('(1+2)*3+4-2-5+2/2*3', assert);
-		testOpExpression('1 + 2-   3*	4 /8', assert);
-		testOpExpression('\n1\r\n+\n2\n', assert);
+	QUnit.module('Ops', function (qunit) {
+		qunit.before(() => {
+			jsep.addBinaryOp('**', 11, true); // ES2016, right-associative
+		});
+		qunit.after(resetJsepDefaults);
+
+		[
+			'1',
+			'1+2',
+			'1*2',
+			'1*(2+3)',
+			'(1+2)*3',
+			'(1+2)*3+4-2-5+2/2*3',
+			'1 + 2-   3*	4 /8',
+			'\n1\r\n+\n2\n',
+			'1 + -2',
+			'-1 + -2 * -3 * 2',
+			'2 ** 3 ** 4',
+			'2 ** 3 ** 4 * 5 ** 6 ** 7 * (8 + 9)',
+			'(2 ** 3) ** 4 * (5 ** 6 ** 7) * (8 + 9)',
+		].forEach(expr => QUnit.test(`Expr: ${expr}`, assert => testOpExpression(expr, assert)));
 	});
 
 	QUnit.test('Custom operators', function (assert) {

test/test_utils.js

@@ -6,6 +6,7 @@ export const binOps = {
 	'*': (a, b) => a * b,
 	'/': (a, b) => a / b,
 	'%': (a, b) => a % b,
+	'**': (a, b) => a ** b, // ES2016
 };
 
 export const unOps = {

typings/tsd.d.ts

@@ -136,11 +136,12 @@ declare module 'jsep' {
 
 		let unary_ops: { [op: string]: any };
 		let binary_ops: { [op: string]: number };
+		let right_associative: Set<string>;
 		let additional_identifier_chars: Set<string>;
 		let literals: { [literal: string]: any };
 		let this_str: string;
 
-		function addBinaryOp(operatorName: string, precedence: number): void;
+		function addBinaryOp(operatorName: string, precedence: number, rightToLeft?: boolean): void;
 
 		function addUnaryOp(operatorName: string): void;