Mastering JavaScript

Timers and callbacks

In implementing timers or callbacks, you need to call the handler asynchronously, mostly at a later point in time. Due to the asynchronous calls, we need to access variables from outside the scope in such functions. Consider the following example:

function delay(message) {
  setTimeout( function timerFn(){
    console.log( message );
  }, 1000 );
}
delay( "Hello World" );

We pass the inner timerFn() function to the built-in library function, setTimeout(). However, timerFn() has a scope closure over the scope of delay(), and hence it can reference the variable message.