Evaluate a string in Reverse Polish Notation and output the result.

polish.js

// https://codegolf.stackexchange.com/questions/221/reverse-polish-notation

'use strict';

// let s = '-4 5 +';
// let s = '5 2 /';
// let s = '5 2.5 /';
// let s = '4 2 5 * +';
// let s = '5 1 2 + 4 * 3 - +';
let s = '4 2 5 * + 1 3 2 * + /';

function evalPostfix(expr /* Array of [num num op] */) {
	switch (expr[2]) {
		case '+':
			return parseFloat(expr[0]) + parseFloat(expr[1]);
			break;
		case '-':
			return parseFloat(expr[0]) - parseFloat(expr[1]);
			break;
		case '/':
			return parseFloat(expr[0]) / parseFloat(expr[1]);
			break;
		case '*':
			return parseFloat(expr[0]) * parseFloat(expr[1]);
			break;
		default:
			// console.log('number');
			break;
	}	
}

var ops = ['+', '-', '*', '/'];

function reversePolish (str) {
	let arr = str.split(' ');

	for (let i = 0, l = arr.length; i < l; i++) {
		if (ops.indexOf(arr[i]) !== -1) {
			let expr = arr.splice(i - 2, 3); // start, deleteCount, itemToInsert
			arr.splice(i - 2, 0, evalPostfix(expr));
			if (arr.length === 1) {
				break;
			} else {
				return reversePolish(arr.join(' '));
			}
		}
	}

	// if we reach the end then we're done
	return arr[0];
}

console.log(reversePolish(s));

// --------------------------------------------------------------------------------

function reversePolish2(str) {
	const dict = {
		'+': (a, b) => a + b,
		'-': (a, b) => a - b,
		'*': (a, b) => a * b,
		'/': (a, b) => a / b
	};
	let stack = [];
	str.split(/\s+/).forEach(ch => {
		if (Object.keys(dict).includes(ch)) {
			let [arg2, arg1] = [stack.pop(), stack.pop()];
			stack.push(dict[ch](parseFloat(arg1, 10), parseFloat(arg2, 10)));
		} else {
			stack.push(ch);
		}
	});
	return stack[0];
}

function assert(a, b) {
	let res = (a === b) ? 'pass!' : `fail: ${a} !== ${b}`;
	console.log(res);
}

assert(reversePolish2('-4 5 +'), 1);
assert(reversePolish2('5 2 /'), 2.5);
assert(reversePolish2('5 2.5 /'), 2);
assert(reversePolish2('4 2 5 * +'), 14);
assert(reversePolish2('5 1 2 + 4 * 3 - +'), 14);
assert(reversePolish2('4 2 5 * + 1 3 2 * + /'), 2);