Functional Programming A Calculator In Javascript

calculator.js

// found this here, OMG http://net.tutsplus.com/tutorials/javascript-ajax/what-they-didnt-tell-you-about-es5s-array-extras

function calculate (calculation) {
    //build an array containing the individual parts
    var parts = calculation.match(
        //   digits    |operators|whitespace
        /(?:\-?[\d\.]+)|[-\+\*\/]|\s+/g
    );
    //test if everything was matched
    if( calculation !== parts.join("") ) {
        throw new Error("couldn't parse calculation")
    }
    //remove all whitespace
    parts = parts.map(Function.prototype.call, String.prototype.trim);
    parts = parts.filter(Boolean);
    //build a separate array containing parsed numbers
    var nums = parts.map(parseFloat);
    //build another array with all operations reduced to additions
    var processed = [];
    for(var i = 0; i < parts.length; i++){
        if( nums[i] === nums[i] ){ //nums[i] isn't NaN
            processed.push( nums[i] );
        } else {
            switch( parts[i] ) {
                case "+":
                    continue; //ignore
                case "-":
                    processed.push(nums[++i] * -1);
                    break;
                case "*":
                    processed.push(processed.pop() * nums[++i]);
                    break;
                case "/":
                    processed.push(processed.pop() / nums[++i]);
                    break;
                default:
                    throw new Error("unknown operation: " + parts[i]);
            }
        }
    }
    //add all numbers and return the result
    return processed.reduce(function(result, elem){
        return result + elem;
    });
}

use.js

calculate(" 2 + 2.5 * 2  ") // returns 7
calculate("12 / 6 + 4 * 3") // returns 14

It's not pure if you push to an array
here's a functional way I made : https://gist.github.com/audrenbdb/94fcaa1e17a8e5db8166c04927259d40