setTimeout

Timeouts can be used to defer the execution of a function until some number of milliseconds into the future.

Consider the following code:

setTimeout(a, 1000);
setTimeout(b, 1001);

One would expect that function b would execute after function a. However, this cannot be guaranteed — a may follow b, or the other way around.

Now, consider the subtle difference present in the following code snippet:

setTimeout(a, 1000);
setTimeout(b, 1000);

The execution order of a and b are predictable in this case. Node essentially maintains an object map grouping callbacks with identical timeout lengths. Isaac Schlueter, a former leader of the Node project and now CEO of npm Inc., puts it in this way:

As we can find on 

The ordering of timer callbacks registered within an identical execution scope does not predictably determine the eventual execution order in all cases. Additionally, there exists a minimum wait time of one millisecond for a timeout. Passing a value of zero, -1, or a non-number will be translated into this minimum value.