What is the difference between setTimeout(fn, 0) and setTimeout(fn, 1)?

setTimeout has a minimum timeout of 4ms. So there is actually no difference between the two.

If the currently running task is a task that was created by the setTimeout() method, and timeout is less than 4, then increase timeout to 4.

Spec

EDIT: As pointed out by Ahmad in the comments, the spec has changed now, so the answer would currently be, "It depends."

I think the answer is "It depends" now.

We can run the code in different platform and browsers:

function setTimeouts() {
  setTimeout(function() { console.log(2); }, 2);
  setTimeout(function() { console.log(1); }, 1);
  setTimeout(function() { console.log(0); }, 0);
}

for (var i = 0; i < 10; i++) {
  setTimeouts();
}

1. For Node.js, 0 is converted to 1, so they are exactly the same: https://github.com/nodejs/node/blob/master/lib/timers.js#L319, and result might be:

    1
    0
    1
    0
    1
    0
    1
    0
    1
    0
    1
    0
    1
    0
    1
    0
    1
    0
    1
    0
    2
    2
    2
    2
    2
    2
    2
    2
    2
    2
   

2. For Chrome, the result is quite similar with Node.js

3. For Firefox, most of 0 will be printed before 1:

    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    1
    1
    1
    1
    1
    1
    1
    1
    1
    1
    2
    2
    2
    2
    2
    2
    2
    2
    2
    2
   

I'm not sure the given answers are correct. Running the following code in Chrome, 0 is clearly invoking the bound function more quickly (just switch the timer values between 0 and 1):

console.log("A");
console.log("B");
var start = new Date().getTime();
setTimeout(function() {
    console.log(new Date().getTime() - start);
}, 0);
console.log("C");
console.log("D");

0 seems to be doing something like Node.js's setImmediate, pushing an instruction onto the end of the current call stack, while 1 invokes whatever the implementation regards as a minimum value.