What is the difference between a function expression vs declaration in JavaScript?

//Function declaration
function foo() { return 5; }

//Anonymous function expression
var foo = function() { return 5; }

//Named function expression
var foo = function foo() { return 5; }

They‘re actually really similar. How you call them is exactly the same, but the difference lies in how the browser loads them into the execution context.

function declarations loads before any code is executed.

While function expressions loads only when the interpreter reaches that line of code.

So if you try to call a function expression before it‘s loaded, you‘ll get an error

But if you call a function declaration, it‘ll always work. Because no code can be called until all declarations are loaded.

ex. Function Expression

alert(foo()); // ERROR! foo wasn‘t loaded yet
var foo = function() { return 5; }

ex. Function Declaration

alert(foo()); // Alerts 5. Declarations are loaded before any code can run.
function foo() { return 5; }

As for the second part of your questions.

var foo = function foo() { return 5; }

is really the same as the other two. It‘s just that this line of code used to cause an error in safari, though it no longer does.