Integer

Problem Link

Problem

Please complete type Integer<T>, which takes a type T and returns the integer part.

type Res1 = Integer<1>        // 1
type Res2 = Integer<1.1>      // 1
type Res3 = Integer<1.9>      // 1
type Res4 = Integer<-1.9>     // -1
type Res5 = Integer<'string'> // never

Solution

type Integer<T extends number | string> =
  `${T}` extends `${infer Int}.${string}`
    ? Int extends `${infer N extends number}`
      ? N
      : never
    : `${T}` extends `${infer N extends number}`
      ? N
      : never

How it works:

1. Convert T to a string template literal.
2. If it contains a ., the part before the dot is the integer portion.
3. If no ., the number is already an integer — return it as-is.
4. Use infer N extends number (TS 4.8+) to parse the string fragment back to a numeric type.
5. Non-numeric strings like 'string' fail the pattern match and return never.

Key Takeaways

• Template literal \${T}`` converts both numbers and numeric strings to a string for pattern matching.
• infer N extends number is the idiomatic way to parse a numeric string back to a number type.
• Splitting on . gives the integer and fractional parts without any arithmetic.