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);
};