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][1]. 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.][2]*

<!-- begin snippet: js hide: false -->

<!-- language: lang-js -->

    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));

<!-- end snippet -->


  [1]: http://stackoverflow.com/a/10050831/575527
  [2]: http://caniuse.com/#feat=arrow-functions