The recursion builds R (an array accumulator type) one element at a time and stops when R['length'] matches N. TypeScript treats tuple length as a numeric literal when the tuple is fully known, which is what makes this comparison possible at the type level. The default R extends T[] = [] lets callers write TupleOf<T, N> without supplying the accumulator. The technique is the building block for every length-aware utility (split, join, take, drop) in advanced TS libraries; understanding it unlocks the rest.

Pairing the type with a constructor closes the loop: callers get a value whose runtime length matches the static length-tracked type. Array.from({ length }, () => value) is the cleanest way to build a fixed-length array of the same value, and the cast to TupleOfV2<T, N> is honest about the static-vs-runtime gap. This shape is what you want for default RGBA, identity-matrix initialisers, and zeroed feature vectors. Use a sparse Array(n) only if every slot will be reassigned before reading; otherwise Array.from keeps the values defined and avoids undefined holes.

LengthOf<T> = T['length'] lifts the runtime length into the type system, which is what makes 'at least N' constraints expressible. Combining it with a literal-type union (extends 0 | 1 | 2) lets the compiler decide a length-based predicate at type level. The same pattern can express at most N, exactly between A and B, or multiple of K (with a bit more recursion). These checks usually surface in libraries that need to reject too-short tuples (route param lists, vector dot-products) at compile time, instead of failing at runtime.