The differences between JavaScript function declarations and function expressions (and why variable hoisting is the reason for these differences).

hoisting.js

// JavaScript code is run in two passes, the first pass 
// does all the function declarations and determines the 
// variables. The second pass actually executes your code.

// Example 1: Function Declaration
// Function declarations will always work because 
// declarations are determined on the first pass and when 
// you actually call the function is performed on the 
// second pass.
foo();
function foo() {
  console.log('Foo ran!');
}

// This works because the function declaration is always 
// handled on the first pass. The interpreter runs this 
// code as if you would have done:
function foo() {
  console.log('Foo ran!');
}
foo();
}


// Example 2: Function Expression
// Function expressions will not always work, because the 
// variable holding that function may be undefined. To truly 
// understand this, we must first understand variable hoisting.

// Because JavaScript runs in two passes, all variable
// assignments are essentially "lifted" to the top of the 
// current scope. For example:
console.log('This is a message.');
var msg = 'Blah blah blah';

// When JavaScript actually runs this code, the first pass
// discovers the variables then the code is run. So it's actually 
// interpreted like this:
var msg;
console.log('This is a message.');
msg = 'Blah blah blah';

// And ths is why function expressions will not always execute 
// whereas function declaration always will. Because when you write 
// this code:
foo();
var foo = function() {
  console.log('Running foo');
};

// It's actually being interpreted like this:
var foo;
foo();  // At this point, foo is "undefined"
foo = function() {
  console.log('Running foo');
};

Good one! Thanks!