recursion in javascript making array, from codeacademy

Question

      I want a recursive function that returns the powers of a number and stores each of them in an array called *stack*.

In other words, each time the recursion is executed, a new value will be added to the stack.

For example, if we called power(3, 3), our stack should end up with the elements [3, 9, 27].

However, the outcome of this code is 27 instead of the array. What is my mistake?

// Create an empty array called "stack"
var stack = [];
// Here is our recursive function
function power(base, exponent) {
    // Base case 
    if ( exponent === 0 ) {
        return 1;
    }
    // Recursive case
    else {
        stack[exponent - 1] = base * power(base, exponent - 1);
        return stack[exponent - 1];
    }
}
power(3,3);

Please edit your code to use proper indentation. It's very difficult to read. — Jordan Running
– Jordan Running, Commented Dec 1, 2016 at 22:20
I appreciate everyone's time and solutions, but I voted for what I can understand. Other codes I need time to study since I am a beginner. Thank you! Also, it has to be a recursion since I am studying recursion but still cannot understand how and when to use it, although I read eloquent javascript several times. — neur.log
– neur.log, Commented Dec 2, 2016 at 3:44

Tolgahan Albayrak · Accepted Answer · 2016-12-01 22:40:11Z

There is no issue with your code. The only issue is the return value. You can see below (BTW, I've minified your code little bit)

var task = (() => {
  var stack = [];
  function power(base, exponent) {
    return (exponent && (stack[--exponent] = base * power(base, exponent))) || 1;
  }
  power(3, 3);
  return stack;
});
console.log(task());

I'm not fan of recursive calls personally and I don't prefer use that pattern unless there is a "have to" case.

So, In your case (Let's forget it is from codeacademy and let's consider that it is kind of real-life case) using a recursive function is not a mandatory.

There are many ways to achieve the same results above.

For example, classic for loop:

function power(base, exp){
  var result = [];
  for(var i=1; i<=exp; i++){
    result.push(Math.pow(base, i));
  }
  return result;
}
console.log(power(3, 3));

Or, ES6 generators (maybe ?)

function *power(base, exp){
  let prev = 1;
  for(var i=0; i<exp; i++){
    yield prev *= base;
  }
}

console.log([...power(3, 3)]);

trincot · Accepted Answer · 2016-12-01 22:36:45Z

Indeed, you don't return the array, but the last calculated product. Actually, the result you are looking for is stored in stack after the call finishes.

But it is bad practice to have a global variable that gets mutated by a function. Instead create a so-called closure for it, and call it powers (plural, to indicate you get an array from it):

function powers(base, exponent) {
    // Create an empty array called "stack"
    var stack = [];
    // Here is our recursive function
    function power(base, exponent) {
        // Base case 
        if ( exponent === 0 ) {
            return 1;
        }
         // Recursive case
        else {
            stack[exponent - 1] = base * power(base, exponent - 1);
            return stack[exponent - 1];
        }
    }
    power(base, exponent);
    return stack;
}
console.log(powers(3,3));

Made a bit more compact:

function powers(base, exponent) {
    var stack = [];
    (function power(exponent) {
        return +!exponent || (stack[--exponent] = base * power(exponent));
    })(exponent);
    return stack;
}
console.log(powers(3,3));

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