@cristiansantana/chile-rut/src/utilities.ts

Javascript (Typescript ready) chilean RUT toolset: Validate, generate and verify format.

import { validateRutIdFormat } from "./validators";
import { getNormalizedRutId } from "./normalizers";

export const getCheckDigit = (rutId: string) => {
    if (!validateRutIdFormat(rutId)) {
        throw new Error("Error: Rut Id has a non valid format");
    }

    const normalizedRutId = getNormalizedRutId(rutId);

    const backwardsRut = normalizedRutId.split("").reverse();

    const factors = [2, 3, 4, 5, 6, 7];

    const sum = backwardsRut.reduce((previous, current, index) => {
        const factorIndex = (factors.length + index) % factors.length;
        return previous + parseInt(current) * factors[factorIndex];
    }, 0);

    const quotient = Math.floor(sum / 11);
    const subtrahend1 = quotient * 11;
    const subtrahend2 = Math.abs(sum - subtrahend1);
    const digit = 11 - subtrahend2;

    if (digit === 11) {
        return "0";
    }
    if (digit === 10) {
        return "K";
    }

    return digit.toFixed(0);
};