Code challenge: validate given string has contains all english alphabet

index.js

// Test if a string uses all letters of the English alphabet.
// "The quick brown fox jumps over the lazy dog."

// Complete a function that tests if a string uses all letters of the alphabet.
const alphabet = [
  "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p",
  "q", "r", "s", "t", "u", "v", "w", "x", "y", "z",
];

// Works but too complex
function solution1(str) {
  const letters = str.toLowerCase().split('');
  const uniqueLetter = Array.from(new Set(letters)).filter(i => alphabet.includes(i)).sort();
  
  return alphabet.join('') === uniqueLetter.join('');
}

function solution2(str) {
  return alphabet.every(i =>  str.toLowerCase().indexOf(i) > -1);
}

function testString(str) {
  // Write your code here!
}

/**
Test code below:
*/

const tests = [
  ["This one should fail", false],
  ["The quick brown fox jumps over The lazy dog.", true],
  ["The five boxing üüü wizards jump quickly", true]
];

const correct = tests.reduce((sum, testCase) => {
  const [input, expectation] = testCase;
  const actual = testString(input);
  if (actual === expectation) {
    return sum && true;
  }
  document.write(
    `With input '${input}', I have expected: '${expectation}', got '${actual}'<br/>`
  );
  return false;
}, true);

if(correct){
  document.write('<b>Everything is correct, thank you</b>')
} else {
  document.write('<b>Please see errors above</b>')
}