This is a infix to postfix expression translator.

infix-to-postfix.js

/*
 * This was created out of my own study
 * of lexical analysis and expression
 * parsing.
 *
 * Demo at: http://dev.shin.cl/infixtopostfix/
 *
 * @author Felipe Alfonso
 */
(function (scope) {
    'use strict';
    var presedence = function (value) {
            if (typeof value === 'undefined') {
                return 0;
            }
            switch (value) {
            case '(':
            case ')':
                return 3;
                break;
            case '*':
            case '/':
                return 2;
                break;
            case '+':
            case '-':
                return 1;
                break;
            default:
                return 0;
                break;
            }
        },
        flush = function (stack, result) {
            while (stack.length) {
                var token = stack.pop();
                if (token !== '(' && token !== ')') {
                    result += token + ' ';
                }
            }
            return result;
        },
        isOperator = function (value) {
            return typeof value === 'string' && value.match(/\+|\-|\*|\/|\(|\)/g) !== null;
        },
        isOperand = function (value) {
            return typeof value === 'string' && value.match(/([a-zA-Z]|[.0-9]+)/g) !== null;
        },
        infixToPostfix = function (expression) {
            var index = 0,
                stack = [],
                result = '',
                length,
                operand = '';

            if (typeof expression !== 'string') {
                return '';
            }
            for (index, length = expression.length; index < length; index++) {
                if (isOperand(expression[index])) {
                    operand = '';
                    while (isOperand(expression[index]) && !isOperator(expression[index])) {
                        operand += expression[index] + '';
                        index++;
                    }
                    index--;
                    result += operand + ' ';
                } else if (isOperator(expression[index])) {
                    if ((presedence(stack[stack.length - 1]) >= presedence(expression[index]) &&
                            stack[stack.length - 1] !== '(') ||
                            expression[index] === ')') {
                        result = flush(stack, result);
                    }
                    stack.push(expression[index]);
                }
            }
            return flush(stack, result);
        };

    scope.infixToPostfix = infixToPostfix;
}(typeof exports !== 'undefined' ? exports : window));