Number Range

Problem Link

Problem

Sometimes we want to limit the range of numbers. For example:

type result = NumberRange<2 , 9>
// | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Solution

Approach: Grow a Tuple and Collect Lengths

Build a tuple from length Low to High, collecting each intermediate length as a union member.

type NumberRange<
  Low extends number,
  High extends number,
  Count extends unknown[] = [],
  Collecting extends boolean = false,
  Result extends number = never
> = Count['length'] extends High
  ? Result | High
  : Collecting extends true
    ? NumberRange<Low, High, [...Count, unknown], true, Result | Count['length']>
    : Count['length'] extends Low
      ? NumberRange<Low, High, [...Count, unknown], true, Result | Low>
      : NumberRange<Low, High, [...Count, unknown], false, Result>

How it works:

1. Increment Count from 0 upward.
2. Once Count['length'] hits Low, set Collecting = true and start adding lengths to Result.
3. When Count['length'] hits High, add High to Result and return.

Key Takeaways

• A Collecting boolean flag avoids checking GreaterThan at every step.
• This pattern — “start collecting at X, stop at Y” — is reusable for any range generation.
• The final | High at the base case ensures the upper bound is inclusive.