Question

It is maybe incredibly easy but I couldn’t solve what was going on.

function doSomething(a)
{
   var num=10;
   return setTimeout(
   function(){ a(num); }, 1000);
}

The only thing that actually confuses me is the a(num) part. What actually it does?
Reminder: I really am asking because I’m not familiar with the javascript syntax.

Answer 1

When the function doSomething() is executed it is passed the parameter a,
a is also some function that is then called when setTimeout() expires after 1 second,
then calling the function a() passing the argument called num

Example usage:

// call doSomething() passing the test() function as an argument
doSomething(test);


// takes a number as an argument and shows an alert with that value
function test(number)
{
   alert(number);
}

// takes a function as an argument that will perform a 1 second timeout then execute the function called a
function doSomething(a)
{
   var num=10;
   return setTimeout(
   function(){ a(num); }, 1000);
}