metropolis/src/items/ean/ean.service.ts

import { Controller } from '@nestjs/common';
import assert from 'assert';

@Controller()
export class EANService {
  // https://en.wikipedia.org/wiki/International_Article_Number#Calculation_of_checksum_digit
  calcChecksum(id: string) {
    assert(id.length === 12);
    let sum = id
      .split('')
      .map((d) => parseInt(d, 10))
      .map((d, i) => d * (i % 2 === 0 ? 1 : 3))
      .reduce((s, d) => s + d, 0);
    while (sum > 10) {
      sum -= 10;
    }
    return 10 - sum;
  }

  isValid(ean: string) {
    return this.calcChecksum(ean.slice(0, 12)) === parseInt(ean[12], 10);
  }

  toID(ean: string) {
    if (ean.length === 12) {
      // it already is an ID
      return ean;
    }
    if (!this.isValid(ean)) {
      throw new Error(`Invalid EAN: "${ean}"`);
    }
    return ean.slice(0, 12);
  }

  fromID(id: string) {
    assert(id.length === 12);
    return id + this.calcChecksum(id);
  }
}