type-samurai/src/round.d.ts

Advanced utility types for Typescript

import { GetFloatNumberParts } from "./get-float-number-parts";
import { IsFloat } from "./number";
import { FirstDigit } from "./first-digit";
import { IsGreaterThan } from "./greater-than";
import { Increment } from "./increment";

/**
 * Type version of `Math.round()`. Returns the value of a number rounded to the nearest integer
 * @example
 * ```ts
 * // 3
 * type T0 = Round<3.14>;
 * // -3
 * type T1 = Round<-3.14>;
 * ```
 */
type Round<T extends number> = IsFloat<T> extends true
  ? GetFloatNumberParts<T> extends [
      infer Whole extends number,
      infer Fraction extends number
    ]
    ? IsGreaterThan<FirstDigit<Fraction>, 4> extends true
      ? Increment<Whole>
      : Whole
    : never
  : T;