I am pretty new to javascript and I have an issue with flattening several arrays into one complete array using the reduce and concat function.

```
var array = [ [1, 3], [6, 6], [3, 3] ]; 

var finishedArray = function(array) {
var reducedArray = {};
var completeArray = {};

for (var i = 0; i < array.length; i++) {
    reducedArray[i] = array[i].reduce(function (a, b) {
        return a + b;
    });

    completeArray = reducedArray[i].concat(reducedArray[i + 1]);
}

return completeArray;

}

console.log(finishedArray(array));`

```

The error I get is : `completeArray = reducedArray[i].concat(reducedArray[i + 1])`;

`TypeError: undefined is not a function`

Why is that???