secondLargestNum.js

//How do you find the second highest number in an integer array?

const secondHighestNumber = arr => {
  if (!Array.isArray(arr) || !arr.length) {
    return null;
  }

  let largestNum = Math.max(...arr);
  arr.splice(arr.indexOf(largestNum), 1);
  let secondHighestNum = Math.max(...arr);

  return secondHighestNum;
};

//Time-complexity: O(n)
//Test cases

const testCase = [
  { input: [], output: null },
  { input: "engimatic", output: null },
  { input: [1, 2, 3, 4, 5, 7, 6], output: 6 },
  { input: [11, 24, 31, 40, 15, 32, 16], output: 32 },
  { input: [-1, 0, 1, 2], output: 1 },
  { input: [-1, -2, -3, -4, -5], output: -2 }
];
testCase.forEach(({ input, output }, index) => {
  console.log(
    `TEST CASE:${index} (${input}):(${
      secondHighestNumber(input) === output ? "success" : "failure"
    })`
  );
});

There is one O(n) solution tho.