Understanding JavaScript Array Storage and Spread Operator

The correct answer to the quiz question is [[1,3], [5,5], [1,8]]. This is achieved by using JavaScript's spread operator and its method of storing array items.

In JavaScript, an array can hold multiple values, and these values can be of any data type including other arrays. In this instance, we are dealing with multi-dimensional arrays. The variable triangle is defined as an array that includes two other arrays (point and an anonymous array [5,5]) and another anonymous array [1,8].

The spread operator (...) is used here to include the values of the segment array into the triangle array. However, it is important to note that the spread operator does not flatten the segment array, but it includes the arrays ("spread" them) as they are.

Let's understand this with the given code:

let point = [1,3], 
segment = [point,[5,5]], 
triangle = [...segment,[1,8]];

In this code:

1. point is an array with two elements [1,3].
2. segment is an array with two elements, where each element itself is an array. The first element is point ([1,3]), and the second element is another array [5,5].
3. triangle is defined using the spread operator (...segment) along with another array [1,8].

Because the spread operator simply "spreads" the segment array elements into the triangle array without flattening them, triangle ends up as being a multi-dimensional array (an array of arrays) as it includes all the constituent arrays of segment along with [1,8].

Hence, triangle is [[1,3], [5,5], [1,8]].

As a best practice, developers should keep a clear understanding of how JavaScript handles arrays and how the spread operator works with multi-dimensional arrays, to avoid any confusion or bugs that might arise. Understanding these concepts properly can also aid in writing more efficient and less error-prone code.