maxOccuringChar.js

//How to find the maximum occurring character in given String?

const maxOccuringChar = str => {
  if (typeof str !== "string" || !str.length) {
    return null;
  }

  const count = {};
  let max = 0;
  let maxChar = null;

  str.split("").forEach(item => {
    if (item !== " ") {
      if (count[item]) {
        count[item] = count[item] + 1;
        if (count[item] > max) {
          max = count[item];
          maxChar = item;
        }
      } else {
        count[item] = 1;
      }
    }
  });

  return maxChar;
};

//Time-complexity: O(n)
//Tests

const testCase = [
  { input: "", output: null },
  { input: 12345, output: null },
  { input: "shilpa", output: null },
  { input: "shilpi", output: "i" },
  { input: "shilpis", output: "i" },
  { input: "h ak un am a ta ta", output: "a" },
  { input: "hakunamatata", output: "a" }
];

testCase.forEach(({ input, output }, index) => {
  console.log(
    `TEST CASE:${index} (${input}):(${
      maxOccuringChar(input) === output ? "success" : "failure"
    })`
  );
});

Perfect.
Two suggestions:

1. Use of for - in is ok in this case but we aware of its limitation.
   https://www.reddit.com/r/javascript/comments/8emf94/forin_vs_objectkeys/dxwecvs?utm_source=share&utm_medium=web2x&context=3

2. Instead of remvoing white-spaces at top-level, you can put that check inside the count-loop (at after line #13), saving one iteration (on line #8).

//How do you swap two numbers without using the third variable?

const swapNumbers = (num1, num2) => {
  if (!Number.isInteger(num1) || !Number.isInteger(num2) || !num1 || !num2) {
    return null;
  }
  num1 = num1 + num2;
  num2 = num1 - num2;
  num1 = num1 - num2;
  console.log(num1, num2);
};

//tests

swapNumbers(1, 2);