Running the following code in the console:
for (var i = 0; i < 5; i++) {
let j = i;
setTimeout(function(){
console.log(j);
}, 0)
}results in some strange behavior. When I run it once, I see something like:
What is that number at the top?
9 // what is this????
0
1
2
3
4
The next time I run it, it's different:
13 // what is this????
0
1
2
3
4
The console will log the result of the final expression that was evaluated. This is why, for example, typing in
'foo' + 'bar'
results in 'foobar' being logged.
Here, the final expression evaluated (synchronously) is the setTimeout, and setTimeout returns an integer indicating the ID of the timeout (which can be used with clearTimeout later). setInterval works the same way. So the number you're seeing is that timeout ID. (it sounds like setTimeout was called 12 times previously on that load of the page)
var foo = 1 would log 1), it's the "result" of the for loop.
setTimeout(). In browsers, the handle is a positive integer that can be passed toclearTimeout()before the callback is scheduled to execute if you want to cancel it.setTimeoutfunction to be stored and used in caseclearTimeoutis needed.