freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/rosetta-code/sedols.english.md

---
title: SEDOLs
id: 59d9c6bc214c613ba73ff012
challengeType: 5
---

## Description
<section id='description'>
    Task:

  <p>
    For each number list of 6-digit
    <a href="https://en.wikipedia.org/wiki/SEDOL" title="wp: SEDOL">SEDOL</a>s,
    calculate and append the checksum digit.
  </p>

  <p>
    That is, given the input string on the left, your function should return the
    corresponding string on the right:
  </p>

    <pre>
     710889 => 7108899
     B0YBKJ => B0YBKJ7
     406566 => 4065663
     B0YBLH => B0YBLH2
     228276 => 2282765
     B0YBKL => B0YBKL9
     557910 => 5579107
     B0YBKR => B0YBKR5
     585284 => 5852842
     B0YBKT => B0YBKT7
     B00030 => B000300
    </pre>

  <p>
    Check also that each input is correctly formed, especially
    with respect to valid characters allowed in a SEDOL string. Your function
    should return <code>null</code> on invalid input.
  </p>
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>sedol</code> is a function.
    testString: assert(typeof sedol === 'function', '<code>sedol</code> is a function.');
  - text: <code>sedol('a')</code> should return null.
    testString: assert(sedol('a') === null, "<code>sedol('a')</code> should return null.");
  - text: <code>sedol('710889')</code> should return '7108899'.
    testString: assert(sedol('710889') === '7108899', "<code>sedol('710889')</code> should return '7108899'.");
  - text: <code>sedol('BOATER')</code> should return null.
    testString: assert(sedol('BOATER') === null, "<code>sedol('BOATER')</code> should return null.");
  - text: <code>sedol('228276')</code> should return '2282765'.
    testString: assert(sedol('228276') === '2282765', "<code>sedol('228276')</code> should return '2282765'.");

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function sedol (input) {
  // Good luck!
  return true;
}
```

</div>



</section>

## Solution
<section id='solution'>


```js
function sedol(input) {
  const checkDigit = sedolCheckDigit(input);
  if (checkDigit !== null) {
    return input + checkDigit;
  }
  return null;
}

const weight = [1, 3, 1, 7, 3, 9, 1];
function sedolCheckDigit(char6) {
  if (char6.search(/^[0-9BCDFGHJKLMNPQRSTVWXYZ]{6}$/) === -1) {
    return null;
  }

  let sum = 0;
  for (let i = 0; i < char6.length; i++) {
    sum += weight[i] * parseInt(char6.charAt(i), 36);
  }
  const check = (10 - (sum % 10)) % 10;
  return check.toString();
}

```

</section>