Sorting - greatest to least

index.js

function greatestToLeast(array) {
  return array.sort(function(a, b) {return b-a;
});
}

/* From here down, you are not expected to 
   understand.... for now :)  
   
   
   Nothing to see here!
   
*/

// tests

function testFunctionWorks(fn, input, expected) {
  const result = fn(input);
  if (
    result &&
    result.length === expected.length &&
    result.every(function(item, index) {
      return index === 0 || result[index] < result[index - 1];
    }) &&
    result.every(function(item) {
      return expected.indexOf(item) > -1;
    })
  ) {
    console.log('SUCCESS: `' + fn.name + '` works!');
    return true;
  } else {
    console.error(
      'FAILURE: `' +
        fn.name +
        '([' +
        input +
        '])` should be ' +
        expected +
        ' but was ' +
        fn(input)
    );
    return false;
  }
}

function runTests() {
  const input1 = [10, 3, 5, 22, 19];
  const result1 = [22, 19, 10, 5, 3];
  const input2 = [2, 4, 6, 8];
  const result2 = [8, 6, 4, 2];

  const testResults = [
    testFunctionWorks(greatestToLeast, input1, result1),
    testFunctionWorks(greatestToLeast, input2, result2),
  ];

  const numPassing = testResults.filter(function(result) {
    return result;
  }).length;
  console.log(numPassing + ' out of ' + testResults.length + ' tests passing.');
}

runTests();