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First, isLt100, isDivBy3 and your other functions are vaguely named. Better name them fully and meaningfully. There's no harm done with the extra keystrokes in the name.

Looking at your fizzbuzz, it appears that loop is impure as it is mutating something from the outside of the function. You're doing a push inside loop to result which is outside the loop. Also, you're mutating i as well.

Also, you would want to make your range flexible. Now, as far as I know, functional programming fans prefer a "range" function to iterate through. This makes it easy for iteration functions like reduce, map, filter etc. to go through. Since there's no such thing in JS, this SO answer provides us one. It's essentially just creating an array of n length.

Next is your fizzbuzz. Since we have an array of numbers thanks to range, all we need to do now is just map these values with numbers, fizz, buzz or fizz buzz. We can use the native map array method to do that. We just need to provide the number to fizzBuzzTest and return it's result to map to create our new array of numbers, fizz, buzz or fizz buzz.

Warning: I'm using ES6 syntax. Run snippet in browser that supports at least arrow functions.

function range(n){
  return Array.apply(null, Array(n)).map((_, i) => i);
}

function fizzBuzzTest(n){
  var by3 = n % 3 === 0;
  var by5 = n % 5 === 0;

  return  by3 && by5 ? 'fizz buzz'
               : by3 ? 'fizz'
               : by5 ? 'buzz'
                     : n;
}

function fizzBuzz(n){
  return range(n).map(x => fizzBuzzTest(x + 1)).join(', ');

  // A non-OOP approach would have the same, except function calls are nested rather
  // than chained (Python)
  // return ','.join(map(lambda x: fizzBuzz(x + 1), range(100)));
}

// SE really needs something elegant to print stuff with on snippets
document.write(fizzBuzz(100));