reverse polish stack

rps.js

var isOp = function (op) {
    return !!({
        '+': true,
        '-': true,
        '*': true,
        '/': true
    }[op])
};
 
var applyOp = function (op, a, b) {
  return ({
        '+': a + b,
        '-': b - a,
        '*': a * b,
        '/': b / a
  })[op];  
};
 
function calc(expr) {
  
  var input = expr.split(' '), 
      len = input.length,
      stack = [], 
      cell;
  
    if (len <= 0) {
        return 0;
    }  
  
    while (input.length > 0) {
        cell = input.shift();
    
        if (!isOp(cell)) {
             stack.push(+cell);        
        } 
        else {
            input.unshift(applyOp(cell, stack.pop(), stack.pop()));
        } 
    }
    return stack.pop();
}