Two ways of declaring functions

All the JavaScript books I read so far do not distinguish between following two ways of declaring a function.

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

Thanks to Ben today I learned that there is a difference .

When a var is used to declare a function then only the variable declaration gets hoisted up

function test() {
  foo();
  var foo = function () {
    console.log("foo");
  };
}
test();

Above code is same as one given below.

function test() {
  var foo;
  foo();
  foo = function () {
    console.log("foo");
  };
}
test();

When a function variable is declared without var then both variable declaration and body gets hoisted up

function test() {
  foo();
  function foo() {
    console.log("foo");
  }
}
test();

Above code is same as one given below.

function test() {
  var foo;
  foo = function () {};
  console.log(foo());
}
test();

Conclusion

Now it will be clear why foo() does not work in the following case while bar() does work.

function test() {
  foo(); // TypeError "foo is not a function"
  bar(); // "this will run!"
  var foo = function () {
    // function expression assigned to local variable 'foo'
    alert("this won't run!");
  };
  function bar() {
    // function declaration, given the name 'bar'
    alert("this will run!");
  }
}
test();