JavaScript (Beginner-Level) Kata - Building a Calculator Using Functions

Question

I'm solving the following kata: Write a program that takes as its first argument one of the words ‘sum,’ ‘product,’ ‘mean,’ or ‘sqrt’ and for further arguments a series of numbers. The program applies the appropriate function to the series.

I have solved it (code below) but it is bulky and inefficient. I'm looking to re-write it have a single function calculator that calls the other functions (i.e. function sum, function product).

My question: how do I write the functions sum, product, sqrt, etc so when called by the function calculator, they properly take the arguments of calculator and compute the math.

Below is the bulky code:

function calculator() {

    var sumTotal = 0;
    var productTotal = 1;
    var meanTotal = 0;
    var sqrt;

    if(arguments[0] === "sum") {
        for(i = 1; i < arguments.length; i++) {
            sumTotal += arguments[i];
        }
    return sumTotal;
    }

    if(arguments[0] === "product") {
        for(i = 1; i < arguments.length; i++) {
            productTotal *= arguments[i];
        }
    return productTotal;
    }

    if(arguments[0] === "mean") {
        for(i = 1; i < arguments.length; i++) {
            meanTotal += arguments[i];
        }
    return meanTotal / (arguments.length-1);
    }

    if(arguments[0] === "sqrt") {
            sqrt = Math.sqrt(arguments[1]);
        }
    return sqrt;

}


calculator("sqrt", 17);

Answer 1

You can just create an object with the functions you need, and then have the calculator function call the correct one.

var operations = {
  sum: function() { /* sum function */ },
  product: function() { /* product function */ },
  mean: function() { /* mean function */ },
  sqrt: function() { /* sqrt function */ }
};

function calculator(operation) {
  operation = operations[operation];
  var args = Array.prototype.slice.call(arguments, 1);
  return operation.apply(this, args);
}

You can see an example of this in action on jsFiddle.

If you don't quite understand what I'm doing in my code, I reccomend reading about call and apply in Javascript and also about objects in Javascript.

Answer 2

You can pass your entire arguments list to another function using the apply() method:

if(arguments[0] === "sum") {
   return sum.apply(this, Array.prototype.slice.call(arguments, 1));
}

With your operations in separate methods:

function sum() {
  var sumTotal = 0;
  for(i = 1; i < arguments.length; i++) {
            sumTotal += arguments[i];
  }
  return sumTotal;
}