Some annotations:

• Return a boolean for a function which starts with is....
• Take the floored middle index without an offset of one.
• Check the string length instead of the middle index without having an integer value.
• Return either the single character or slice the string with adjustmens for start and end.

function isOdd(num) {
  return num % 2 === 1;
}

function getMiddle(str) {
    const middleDigit = Math.floor(str.length / 2);
    return isOdd(str.length)
        ? str[middleDigit]
        : str.slice(middleDigit - 1, middleDigit + 1);
}

console.log(getMiddle('the'));
console.log(getMiddle('boot'));

you are passing the wrong value to is_odd

function isOdd(num) {
    return ((num % 2) === 1);
}

function getMiddle(str) {
    const middleDigit = Math.floor(str.length / 2);
    if (isOdd(str.length)) {
        return str[middleDigit];
    } else {
        return str[middleDigit - 1] + str[middleDigit];
    }
}
console.log(getMiddle(`they`));

Your execution was a little bit off!

• I changed your isOdd function to return a boolean value, instead of a number.
• After every calculation of the middle digit I subtracted 1 from the result, as we are working with indexes (and they start the count of the positions at 0, instead of 1).
• For the second middle digit when the word length is even, you just have to do "str.length/2", no need for adding or subtracting 1.

    function isOdd(num) {
      return num % 2 === 1;
    }
    
    function getMiddle(str) {
      if (isOdd(str.length)) {
        return str[((str.length + 1) / 2) - 1];
      } else {
        return str[(str.length / 2) - 1] + str[str.length / 2];
      }
    }
    console.log(getMiddle(`the`));
    console.log(getMiddle(`root`));