Chunky Monkey Question abpit array.push()

I have a question about array.push() method.

Can someone explain the difference in these two lines of code.
//Line 1:
// testArr = myArray.push(arr.slice(i,(size+i)));

//Line 2:     
  testArr.push(arr.slice(i, (size+i)));

Link to Code:

I tried numerous different ways to come to the solution but just stumbled onto the answer almost by accident.

.push returns the new length of the modified array after pushing.

In line 1, testArr has the value of myArray's updated size.
In line 2, testArr is modified to have a new element as it’s last value.