Alternative to `Math.frac()` in Javascript

frac.spec.ts

// testing using Jest
import { frac } from './frac';

describe('Math frac tests', () => {
    it('should be defined', () => {
        expect(frac).toBeDefined();
    });

    it('should accurately calculate fraction of a number', () => {
        expect( frac(-3.1234) ).toEqual( -0.1234 );
        expect( frac(1.234) ).toEqual( 0.234 );
        expect( frac(56789e-3) ).toEqual( 0.789 );
        expect( frac(12345678) ).toEqual( 0 );
        expect( frac(-34.5697) ).toEqual( -0.5697 );
    });
});

frac.ts

/**
 * TBasically Math.frac()
 * Alternative to less accurate hacks:
 * ```ts
 * const fracNum = (num: number) => num % 1;
 * console.log( fracNum(34.5697) ) //  0.5696999999999974 in Node
 * ```
 * @example const foo = frac(34.5697) // 0.569
 * 
 * LICENCE: MIT
 */
export const frac = (num: number) =>
    +(num.toString()).replace(
        Math.trunc(num).toString(),
        '0'
    ) * Math.sign(num);