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6kyu/find-the-missing-letter/description.md

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+Find the missing letter
+Write a method that takes an array of consecutive (increasing) letters as input and that returns the missing letter in the array.
+
+You will always get an valid array. And it will be always exactly one letter be missing. The length of the array will always be at least 2.
+The array will always contain letters in only one case.
+
+Example:
+
+```
+['a','b','c','d','f'] -> 'e'
+['O','Q','R','S'] -> 'P'
+```
+
+(Use the English alphabet with 26 letters!)
+
+Have fun coding it and please don't forget to vote and rank this kata! :-)
+
+I have also created other katas. Take a look if you enjoyed this kata!

6kyu/find-the-missing-letter/solution.ts

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+export function findMissingLetter(array: string[]): string {
+  const alphabet = 'abcdefghijklmnopqrstuvwxyz';
+  // convert array to alphabet index values
+  const indexes = array.map((char) => alphabet.indexOf(char.toLowerCase()));
+  // find the missing index
+  const missingIndex = indexes.find((index, i) => indexes[i + 1] - index > 1) as number;
+
+  const isInputUpperCase = array[0] === array[0].toUpperCase();
+  const missingLetter = alphabet[missingIndex + 1];
+  return isInputUpperCase ? missingLetter.toUpperCase() : missingLetter;
+}

6kyu/find-the-missing-letter/tests.ts

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+import { findMissingLetter } from './solution';
+import { assert } from "chai";
+
+describe("solution", function(){
+  it("exampleTests", function(){
+    assert.equal(findMissingLetter(['a','b','c','d','f']), 'e');
+    assert.equal(findMissingLetter(['O','Q','R','S']), 'P');
+  });
+});