Tuples, rest parameters, and array boundaries

The idea

A tuple describes a fixed arrangement of positions, such as a label followed by a count. A general array describes repeated elements but does not establish that a particular position exists. Tuple labels improve readability without changing runtime behavior. Rest parameters can use a tuple to connect each argument position to its intended type.

An optional tuple element can be absent, so reading it includes undefined under strict checking. A readonly tuple restricts writes through that view but still represents an ordinary JavaScript array at runtime. For general array indexing, enabling noUncheckedIndexedAccess adds undefined to reads that may be out of bounds. Regardless of compiler options, runtime algorithms must still check lengths or handle absent values.

function line(...[name, count = 1]: [name: string, count?: number]): string {
  return name + ":" + count;
}
console.log(line("item"));
console.log(line("item", 0));

item:1
item:0

The optional count defaults only when undefined, preserving zero.