Explain the differences on the usage of foo between function foo() {} and var foo = function() {}

The former is a function declaration while the latter is a function expression. The key difference is that function declarations have its body hoisted but the bodies of function expressions are not (they have the same hoisting behavior as variables). If you try to invoke a function expression before it is defined, you will get an Uncaught TypeError: XXX is not a function error.

Function Declaration

foo(); // 'FOOOOO'
function foo() {
  console.log('FOOOOO');
}

Function Expression

foo(); // Uncaught TypeError: foo is not a function
var foo = function() {
  console.log('FOOOOO');
};