feat(isDigit): add isDigit type

src/string/isDigit/algo.ts

@@ -0,0 +1,14 @@
+import type { IsStringLiteral } from "@predicates"
+
+export type _IsDigit<T> = [T] extends [
+  `${number}`,
+]
+  ? true
+  : false
+
+export type IsDigit_Back<T> =
+  IsStringLiteral<T> extends true
+    ? [T] extends [`${number}`]
+      ? true
+      : false
+    : false

src/string/isDigit/index.test.ts

@@ -0,0 +1,88 @@
+import {
+  describe,
+  it,
+  expectTypeOf,
+} from "vitest"
+
+import type {
+  _IsDigit,
+  IsDigit_Back,
+} from "./algo"
+
+describe("_IsDigit type tests", () => {
+  it("should return true for string literals that match numeric patterns", () => {
+    type T1 = _IsDigit<"0"> // "0" extends `${number}` => true
+    type T2 = _IsDigit<"123"> // "123" extends `${number}` => true
+    type T3 = _IsDigit<"001"> // "001" extends `${number}` => true
+    type T4 = _IsDigit<"1.2"> // "1.2" extends `${number}` => true (valid numeric string)
+    expectTypeOf<T1>().toEqualTypeOf<true>()
+    expectTypeOf<T2>().toEqualTypeOf<true>()
+    expectTypeOf<T3>().toEqualTypeOf<true>()
+    expectTypeOf<T4>().toEqualTypeOf<true>()
+  })
+
+  it("should return false for string literals that are not numeric", () => {
+    type T1 = _IsDigit<""> // empty string => not a valid number
+    type T2 = _IsDigit<"abc"> // not a numeric string
+    type T3 = _IsDigit<"1a2"> // contains non-numeric characters
+    type T4 = _IsDigit<"one"> // obviously not numeric
+    expectTypeOf<T1>().toEqualTypeOf<false>()
+    expectTypeOf<T2>().toEqualTypeOf<false>()
+    expectTypeOf<T3>().toEqualTypeOf<false>()
+    expectTypeOf<T4>().toEqualTypeOf<false>()
+  })
+
+  it("should return false for non-literal or non-string types", () => {
+    // For a generic string, TS cannot confirm it's strictly `${number}` literal.
+    type T1 = _IsDigit<string>
+    type T2 = _IsDigit<number> // definitely not a string literal
+    type T3 = _IsDigit<123> // same, not a string
+    expectTypeOf<T1>().toEqualTypeOf<false>()
+    expectTypeOf<T2>().toEqualTypeOf<false>()
+    expectTypeOf<T3>().toEqualTypeOf<false>()
+  })
+})
+
+describe("IsDigit_Back type tests", () => {
+  it("should return true for numeric string literals", () => {
+    type T1 = IsDigit_Back<"0">
+    type T2 = IsDigit_Back<"123">
+    type T3 = IsDigit_Back<"001">
+    type T4 = IsDigit_Back<"1.2">
+    expectTypeOf<T1>().toEqualTypeOf<true>()
+    expectTypeOf<T2>().toEqualTypeOf<true>()
+    expectTypeOf<T3>().toEqualTypeOf<true>()
+    expectTypeOf<T4>().toEqualTypeOf<true>()
+  })
+
+  it("should return false for non-numeric or non-literal strings", () => {
+    type T1 = IsDigit_Back<"abc"> // not numeric
+    type T2 = IsDigit_Back<"1a2"> // contains letters
+    type T3 = IsDigit_Back<""> // empty string => not a valid number
+    expectTypeOf<T1>().toEqualTypeOf<false>()
+    expectTypeOf<T2>().toEqualTypeOf<false>()
+    expectTypeOf<T3>().toEqualTypeOf<false>()
+  })
+
+  it("should return false for generic string or non-string types", () => {
+    type T1 = IsDigit_Back<string> // not a literal
+    type T2 = IsDigit_Back<number> // not a string at all
+    type T3 = IsDigit_Back<123> // also not a string
+    expectTypeOf<T1>().toEqualTypeOf<false>()
+    expectTypeOf<T2>().toEqualTypeOf<false>()
+    expectTypeOf<T3>().toEqualTypeOf<false>()
+  })
+
+  it("should return false when given a union of string literals (one not digit)", () => {
+    // If IsStringLiteral is strictly checking for a single literal type,
+    // then a union of two string literals is not "a single literal."
+    type T1 = IsDigit_Back<"123" | "abc" | "234">
+    expectTypeOf<T1>().toEqualTypeOf<false>()
+  })
+  it("should return true when given a union of string literals (all digits)", () => {
+    // If IsStringLiteral is strictly checking for a single literal type,
+    // then a union of two string literals is not "a single literal."
+    type T1 = IsDigit_Back<"123" | "345" | "1214">
+    expectTypeOf<T1>().toEqualTypeOf<true>()
+  })
+})

src/string/isDigit/index.ts

@@ -0,0 +1,11 @@
+/**
+ * A type that returns true if a param is a stringified number, and false otherwise.
+ *
+ * @template Str - The string literal type to reverse.
+ * @returns {string} - Returns the reversed string.
+ */
+
+import type { _IsDigit } from "./algo"
+
+export type IsDigit<T extends string> =
+  _IsDigit<T>