Why doesn’t setTimeout(.., 0) execute immediately?

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Javascript code runs only on one thread. setTimeout schedules a function to run later. So in js when all currently running code finish its execution , event loop will look for any other event.
So setTimeout( .. 0) will make code run after the current loop.

console.log("I'm message from outside timeout"); will be first scheduled to executued. As soon as it finish the setTimeout will be executed

So bottom line setTimeout(myfunction ,0) will run myfunction 0ms after currently executing function. & in your case the current execution loop is 

console.log("I'm message from outside timeout");

If you add another console.log(“I’m message from outside timeout1”);
so current event loop will first log 

I'm message from outside timeout
I'm message from outside timeout1

before starting setTimeout function.

NOTE setTimeout has a minimum timeout of 4ms . You can look at this Stackoverflow thread to know more about it

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