How to retrieve an element from a jagged array given its true index?

Question

If one has a jagged array and the "true index" of an element, which is just the index of the element if the array was flattened (coined by me, I don't know what that's called), what is the fastest way to retrieve the element, given it's index?

For example, take this array:

[[0, 1, 2, 3, 4],
 [5, 6, 7, 8, 9, 10, 11, 12, 13],
 [14, 15],
 [16, 17, 18]]

The value 11 in this jagged 2D array would have an index of 11. What is the fastest and cleanest way to retrieve the value of an element in a jagged array (let's say it's 2D if that makes it easier) given it's "true index"?

Would appreciate a javascript answer, but as this is a general algorithmic question, I don't care what language it is in. If you will provide an answer that is not in javascript, perhaps don't use libraries/modules that wouldn't be available in javascript.

Answer 1

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