This is a Typescript implementation of a gapped buffer.
A gapped buffer is an extensible array (like a normal Javascript Array) except that inserts and deletes behave with amoritized constant time rather than linear. This means that walks over an array doing O(N) inserts and deletes run in linear time rather than O(N^2).
A typical extensible array maintains empty space only at the end which means that inserts or deletes anywhere else require shifting O(N) elements. A gapped buffer maintains the empty space near where the last insert or delete occurred. Therefore inserts or deletes with locality are very cheap.
Reading or writing existing values do not require moving the gap and have performance equivalent to a standard Array.
A gapped buffer's performance can degrade if the gap "thrashes" - that is if inserts and deletes exhibit no locality. Of course, this is no worse than a standard Array but might be worse than more complex linked data structures that are better tuned for that usage.
The gapped buffer has operations that match semantics and signature of default Array operations to make replacement straightforward.