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PatrickMcSweeny's solution

to Luhn in the JavaScript Track

Published at Aug 20 2019 · 0 comments
Instructions
Test suite
Solution

Given a number determine whether or not it is valid per the Luhn formula.

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.

The task is to check if a given string is valid.

Validating a Number

Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.

Example 1: valid credit card number

4539 1488 0343 6467

The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling

4_3_ 1_8_ 0_4_ 6_6_

If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:

8569 2478 0383 3437

Then sum all of the digits:

8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

Example 2: invalid credit card number

8273 1232 7352 0569

Double the second digits, starting from the right

7253 2262 5312 0539

Sum the digits

7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57

57 is not evenly divisible by 10, so this number is not valid.

Setup

Go through the setup instructions for Javascript to install the necessary dependencies:

https://exercism.io/tracks/javascript/installation

Requirements

Install assignment dependencies:

$ npm install

Making the test suite pass

Execute the tests with:

$ npm test

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by changing xtest to test.

Source

The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

luhn.spec.js

import { Luhn } from './luhn';

describe('Luhn', () => {
  test('single digit strings can not be valid', () => {
    const luhn = new Luhn('1');
    expect(luhn.valid).toEqual(false);
  });

  xtest('a single zero is invalid', () => {
    const luhn = new Luhn('0');
    expect(luhn.valid).toEqual(false);
  });

  xtest('a simple valid SIN that remains valid if reversed', () => {
    const luhn = new Luhn('059');
    expect(luhn.valid).toEqual(true);
  });

  xtest('a simple valid SIN that becomes invalid if reversed', () => {
    const luhn = new Luhn('59');
    expect(luhn.valid).toEqual(true);
  });

  xtest('a valid Canadian SIN', () => {
    const luhn = new Luhn('055 444 285');
    expect(luhn.valid).toEqual(true);
  });

  xtest('invalid Canadian SIN', () => {
    const luhn = new Luhn('055 444 286');
    expect(luhn.valid).toEqual(false);
  });

  xtest('invalid credit card', () => {
    const luhn = new Luhn('8273 1232 7352 0569');
    expect(luhn.valid).toEqual(false);
  });

  xtest('valid number with an even number of digits', () => {
    const luhn = new Luhn('095 245 88');
    expect(luhn.valid).toEqual(true);
  });

  xtest('valid strings with a non-digit included become invalid', () => {
    const luhn = new Luhn('055a 444 285');
    expect(luhn.valid).toEqual(false);
  });

  xtest('valid strings with a non-digit added at the end invalid', () => {
    const luhn = new Luhn('059a');
    expect(luhn.valid).toEqual(false);
  });

  xtest('valid strings with punctuation included become invalid', () => {
    const luhn = new Luhn('055-444-285');
    expect(luhn.valid).toEqual(false);
  });

  xtest('valid strings with symbols included become invalid', () => {
    const luhn = new Luhn('055£ 444$ 285');
    expect(luhn.valid).toEqual(false);
  });

  xtest('single zero with space is invalid', () => {
    const luhn = new Luhn(' 0');
    expect(luhn.valid).toEqual(false);
  });

  xtest('more than a single zero is valid', () => {
    const luhn = new Luhn('0000 0');
    expect(luhn.valid).toEqual(true);
  });

  xtest('input digit 9 is correctly converted to output digit 9', () => {
    const luhn = new Luhn('091');
    expect(luhn.valid).toEqual(true);
  });

  xtest('strings with non-digits is invalid', () => {
    const luhn = new Luhn(':9');
    expect(luhn.valid).toEqual(false);
  });
});
const add = (accumulator, currentValue) => accumulator + currentValue;

export class Luhn {
  constructor(input) {
    this.numbers = input
      .replace(/\s/g, "")
      .split("")
      .reverse()
      .map(char => parseInt(char));
    this.multiplyEvens();
    this.subtractNine();
  }

  get valid() {
    if (this.numbers.length < 2)
      return false;

    return this.numbers.reduce(add) % 10 == 0;
  }

  multiplyEvens() {
    this.numbers = this.numbers.map(function(number, i) {
      return i % 2 !== 0 ? number * 2 : number;
    });
  }

  subtractNine() {
    this.numbers = this.numbers.map(function(number) {
      return number > 9 ? number - 9 : number;
    });
  }
}

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