setTimeout(myFunction, 3000);// if you have defined a function named myFunction it will run after 3 seconds (3000 milliseconds)

function test1()
{

   //some code

   //Let's say JS did have a sleep function.
   sleep(3000); //sleep for 3 seconds,

   alert('hi');
}

function test2()
{
    setTimeout(function() { alert('hello');}, 3000); //defer the execution of anonymous function for 3 seconds and go to next line of code.

    alert('hi');
}