Function expressions

As the function creation happens in the context of the assignment expression (to the right side of =), this is a Function Expression:
```
let sayHi = function() {
	alert( "Hello" );
};

// note that Function Expressions have a semicolon `;` at the end
```
Please note, there’s no name after the function keyword. Omitting a name is allowed for Function Expressions.
Here we immediately assign it to the variable, so the meaning of these code samples is the same: "create a function and put it into the variable sayHi".
No matter how the function is created, a function is a value.

Best example to demonstrate function expressions:

function ask(question, yes, no) {
  if (confirm(question)) yes()
  else no();
}

function showOk() {
  alert( "You agreed." );
}

function showCancel() {
  alert( "You canceled the execution." );
}

// usage: functions showOk, showCancel are passed as arguments to ask
ask("Do you agree?", showOk, showCancel);

The arguments showOk and showCancel of ask are called callback functions or just callbacks.

Function Expression	Function Declaration
A Function Expression is created when the execution reaches it and is usable only from that moment.	A Function Declaration can be called earlier than it is defined.
When a Function Expression is within a code block, it’s visible everywhere.	In strict mode, when a Function Declaration is within a code block, it’s visible everywhere inside that block. But not outside of it.