Trunc

In this medium-level challenge, you'll implement a type-level version of Math.trunc that takes a string or number type and returns the integer part by removing any fractional digits.

[object Object]

type cases = [
  Expect<Equal<Trunc<0.1>, '0'>>,
  Expect<Equal<Trunc<0.2>, '0'>>,
  Expect<Equal<Trunc<1.234>, '1'>>,
  Expect<Equal<Trunc<12.345>, '12'>>,
  Expect<Equal<Trunc<-5.1>, '-5'>>,
  Expect<Equal<Trunc<'.3'>, '0'>>,
  Expect<Equal<Trunc<'1.234'>, '1'>>,
  Expect<Equal<Trunc<'-.3'>, '-0'>>,
  Expect<Equal<Trunc<'-10.234'>, '-10'>>,
  Expect<Equal<Trunc<10>, '10'>>,
]

Detailed Explanation

type Trunc<T extends string | number> =
  `${T}` extends `${infer Int}.${string}`
    ? Int extends '' | '-'
      ? `${Int}0`
      : Int
    : `${T}`;

How it works:

• The input T is first converted to a string template literal via `${T}`, which normalizes both number and string inputs into a string representation
• The conditional type checks if the stringified value contains a decimal point using the pattern `${infer Int}.${string}`, where Int captures everything before the dot and string discards the fractional part
• If there is a decimal point, the integer part Int is returned -- but with a special check: if Int is empty (e.g., input .3) or just a minus sign (e.g., input -.3), we append 0 to produce '0' or '-0' respectively
• If there is no decimal point, the entire stringified value is returned as-is since it is already an integer

This challenge helps you understand template literal type inference and string parsing at the type level, and how to apply these concepts in real-world scenarios.