Code example

Question: What is the output of code below and why?

setTimeout(function() {
    console.log("A");
}, 1000);

setTimeout(function() {
    console.log("B");
}, 0);

getDataFromDatabase(function(err, data) {
    console.log("C");
    setTimeout(function() {
        console.log("D");
    }, 1000);
});

console.log("E");

Output: This is known for sure: EBAD. C is unknown when it will be logged.

Explanation: The compiler will not stop on the setTimeout and the getDataFromDatabase methodes. So the first line he will log is E. The callback functions (first argument of setTimeout**) will run after the set timeout on a asynchronous way!

More details:

1. E has no setTimeout
2. B has a set timeout of 0 milliseconds
3. A has a set timeout of 1000 milliseconds
4. D must request a database, after it must D wait 1000 milliseconds so it comes after A.
5. C is unknown because it is unknown when the data of the database is requested. It could be before or after A.