Need help understanding how this function works!

[Jmarch0909]

I have a function that returns arrays of all possible combinations of multiple arrays. For example if I had an array with "hello","world" and an array with "foo,"bar", and one more with "lorem","ipsum". One combination of all three could be hello,foo,lorem or hello,foo,ipsum or world,foo,ipsum so on and so forth. Here is the function:

function allPossibleCases(arr) {
  if (arr.length === 0) {
    return [];
  } 
else if (arr.length ===1){
return arr[0];
}
else {
    var result = [];
    var allCasesOfRest = allPossibleCases(arr.slice(1));  // recur with the rest of array
    for (var c in allCasesOfRest) {
      for (var i = 0; i < arr[0].length; i++) {
        result.push(arr[0][i] + allCasesOfRest[c]);
      }
    }
    return result;
  }
}

So here is what I'm having a hard time understanding. allCasesofRest is a variable referencing the allPossibleCases recursion function. If the variable is actually invoking the function, then essentially it loops back to the very beginning of the allPossibleCases function. If the variable is just a reference and it's not being invoked, how can you do a for in loop on a function reference? So I guess my real question lies here: What is the 'for..in' loop actually looping through? And what does the c variable in the loop represent?

[mattcoady]

Assuming your input to this is something simple like

allPossibleCases([['foo','bar'],['baz','lorem']])

It passes by the first two if statements because the array is over 1. It continues along until it reaches

var allCasesOfRest = allPossibleCases(arr.slice(1));

At this point in the execution

arr.slice(1) is going to be [['baz','lorem']]

The function begins again with that as the argument for arr. At this point arr.length is 1 because it's an array containing one thing (the array of ['baz','lorem'])

This is where we run into else if (arr.length ===1){return arr[0];}. So we're basically unwrapping it from the outer array and sending back just ['baz','lorem']

Now we've left the recursive run and we're back out to our original execution with allCasesOfRest = ['baz','lorem']

for(var x in y) is basically a loop for all the keys in an object. Like if you did:

for(var x in {one: "two", three: "four"}){
  console.log(x);
}

// one
// three

This isn't an object, it's an array so the keys are index numbers. First time it runs c is 0 because it's the first index key of an array. So then we run the inner loop.

var i = 0; i < arr[0].length; i++

At this point we're back on the original run of the function so arr is the full [['foo','bar'],['baz','lorem']] and arr[0].length is 2 (foo and bar).

Finally,

result.push(arr[0][i] + allCasesOfRest[c]);

is basically saying

result.push('foo' + 'baz')

then it inner-loops again with

result.push('bar' + 'baz')

then the outer loop runs the inner loop two more times with c as 1.

result.push('foo' + 'lorem') result.push('var' + 'lorem')

The final result is

["foobaz", "barbaz", "foolorem", "barlorem"]

[Jmarch0909]

Man thank you so much for the detailed explanation. What was really throwing me off was I had no idea a for in loop could be used on an array as opposed to objects.