add string to number

string-to-number.ts

@@ -0,0 +1,31 @@
+import type { Equal, Expect } from '@type-challenges/utils'
+
+type cases = [
+  Expect<Equal<ToNumber<'0'>, 0>>,
+  Expect<Equal<ToNumber<'5'>, 5>>,
+  Expect<Equal<ToNumber<'12'>, 12>>,
+  Expect<Equal<ToNumber<'27'>, 27>>,
+  Expect<Equal<ToNumber<'18@7_$%'>, never>>,
+]
+
+type Digit = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9';
+
+type IsNumber<
+  S extends string,
+> =
+S extends `${Digit}${infer Tail}`
+  ? IsNumber<Tail>
+  : S extends ''
+    ? true
+    : false
+;
+
+type ToNumber<
+  S extends string,
+  Acc extends 1[] = []
+> =
+IsNumber<S> extends true
+  ? `${Acc['length']}` extends S
+    ? Acc['length']
+    : ToNumber<S, [...Acc, 1]>
+  : never