Integer

In this medium-level challenge, you'll implement an Integer<T> type that checks whether a numeric type T is an integer, returning T if it is and never otherwise.

Detailed Explanation

type Integer<T extends number> =
  number extends T
    ? never
    : `${T}` extends `${bigint}`
      ? T
      : never;

How it works:

• The first check number extends T handles the case where T is the wide number type (not a specific numeric literal). If T is just number, we return never since we cannot determine if an arbitrary number is an integer.
• For specific numeric literals, we convert T to a string via template literal \${T}`and check if it matches the pattern`${bigint}``.
• The bigint template pattern matches strings that represent integers (like "1", "28", "-5") but does not match strings with decimal points (like "1.1", "0.5").
• Values like 1.0 are normalized by TypeScript to 1, so \${1.0}`becomes"1", which correctly matches the bigint` pattern.

An alternative approach uses explicit decimal point detection:

type Integer<T extends number> =
  number extends T
    ? never
    : `${T}` extends `${string}.${string}`
      ? never
      : T;

This alternative checks if the string representation contains a decimal point, but note it would also accept scientific notation numbers like 3e23 as integers. The bigint approach correctly rejects these since "3e+23" does not match bigint.

This challenge helps you understand template literal types and type narrowing with numeric types, and how to apply these techniques in real-world scenarios.