oneDuplicateNum.js

//In an array 1-100 exactly one number is duplicate how do you find it?

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

  let count = {};

  for (let i = 0; i < arr.length; i++) {
    if (count[arr[i]]) {
      return arr[i];
    } else {
      count[arr[i]] = true;
    }
  }
  return null;
};

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

const testCase = [
  { input: [], output: null },
  { input: ["a", "b", "c"], output: null },
  { input: "engimatic", output: null },
  { input: [1, 2, 3, 4, 5, 2, 6], output: 2 },
  { input: [1, 2, 3, 4, 5, 3, 6], output: 3 },
  { input: [1, 2, 3, 4, 5, 6, 6], output: 6 },
  { input: [1, 1, 3, 4, 5, 2, 6], output: 1 }
];
testCase.forEach(({ input, output }, index) => {
  console.log(
    `TEST CASE:${index} (${input}):(${
      findOneDuplicateNum(input) === output ? "success" : "failure"
    })`
  );
});

Perfect!

Thank you.