Thousand separator.

Separate.d.ts

// 判断类型是否相等
type Equal<X, Y> = (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y ? 1 : 2 ? true : false

// 条件判断
type If<C extends boolean, T, F> = C extends true ? T : F

// 获取类型数组最后一个类型
type LastEl<T extends string[]> = T extends [...infer _P, infer K] ? K : never

// 合法数字表达集合
type Numeric = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' | '-' | '.'

// 字符串转为字符数组（反转）
type StrToArr<T extends string, R extends string[] = []> = T extends `${infer A}${infer B}` ? StrToArr<B, [A, ...R]> : R

// 字符串长度
type StrLen<T extends string> = StrToArr<T>['length']

// 字符数组转字符串
type JoinStr<T extends string[], R extends string = ''> = T extends [...infer A, infer B]
  ? `${Extract<B, string>}${JoinStr<Extract<A, string[]>, R>}`
  : R

// 判断负号（-）位置是否合法
type IsValidPrefix<Char extends string, Idx extends number> = Char extends '-' ? Equal<Idx, 0> : true

// 判断小数点（.）是否合法
type IsValidDot<
  Char extends string,
  CharArr extends string[],
  DotArr extends string[],
  Len extends number,
> = Char extends '.'
  ? If<
      Equal<CharArr['length'], 0>,
      false,
      If<
        Equal<LastEl<CharArr>, '-'>,
        false,
        If<Equal<[...CharArr, Char]['length'], Len>, false, Equal<DotArr['length'], 0>>
      >
    >
  : true

// 判断是否为合法数字表达字符串
type IsNumericStr<
  Str extends string,
  CharArr extends string[] = [],
  DotArr extends string[] = [],
  Len extends number = StrLen<Str>,
> = Str extends `${infer Char}${infer RestStr}`
  ? Char extends Numeric
    ? If<
        Equal<IsValidPrefix<Char, CharArr['length']> & IsValidDot<Char, CharArr, DotArr, Len>, true>,
        IsNumericStr<RestStr, [...CharArr, Char], If<Equal<Char, '.'>, [...DotArr, '.'], DotArr>, Len>,
        false
      >
    : false
  : If<Equal<Len, 0>, false, If<Equal<Len, 1>, If<Equal<Str, '.'>, false, If<Equal<Str, '-'>, false, true>>, true>>

// 字符串分隔
type NumSeparate<T extends string[], R extends string = ''> = T extends [infer A, infer B, infer C, ...infer D]
  ? `${NumSeparate<
      Extract<D, string[]>,
      `${Extract<C, string>}${Extract<B, string>}${Extract<A, string>}${If<Equal<R, ''>, '', `,${R}`>}`
    >}`
  : If<Equal<T['length'], 0>, R, `${JoinStr<T>}${If<Equal<R, ''>, '', `,${R}`>}`>

type Separate<T extends string> = If<
  IsNumericStr<T>,
  T extends `-${infer A}.${infer B}`
    ? `-${NumSeparate<StrToArr<A>>}.${B}`
    : T extends `-${infer B}`
    ? `-${NumSeparate<StrToArr<B>>}`
    : T extends `${infer C}.${infer D}`
    ? `${NumSeparate<StrToArr<C>>}.${D}`
    : NumSeparate<StrToArr<T>>,
  never
>

type A = Separate<'121241'> // 121,241
type B = Separate<'121342.41'> // 121,342.41
type C = Separate<'-3526235236'> // -3,526,235,236
type D = Separate<'-2213a'> // never
type E = Separate<'-35262352.36'> // -35,262,352.36