Indentation:

You're missing indentation in your code, you should space out each level of indentation.

For example:

for (var n = 1; n <= 100; n++) {
var output = "";
if (n % 3 == 0)
    output += "fizz";

would become:

for (var n = 1; n <= 100; n++) {
    var output = "";
    if (n % 3 == 0)
        output += "fizz";

Use braces

You should wrap your code in braces, so you don't cause any bugs unknowingly:

For example, you (or someone in future that maintains your code) might think this wouldn't log anything, however, "omg" is logged.

if (5 != 5)
    console.log("wow");
    console.log("omg")

Even Apple got caught out by one of these bugs.

Unnecessary logic:

In your block, you check whether the number is divisible by 3, then by 5, then by 3 and 5. You don't need to perform the last check of 3 and 5, the first two make up that one.

Leaving you with:

Implementing all those changes, you would end up with:

for (var n = 1; n <= 100; n++) {
    var output = "";
    if (n % 3 == 0){
        output += "fizz";
    }
    if (n % 5 == 0){
        output += "buzz";
    }
    console.log(output || n);
};