What is the scope of variables in JavaScript?

JavaScript programmers are practically ranked by how well they understand scope.

1. A globally-scoped variable

var a = 1;
// global scope
function one() {
  alert(a); // alerts '1'
}

2. Local scope

var a = 1;
function two(a) {
  alert(a); // alerts the given argument, not the global value of '1'
}
// local scope again
function three() {
  var a = 3; // alerts '3'
  alert(a);
}

3. Intermediate: No such thing as block scope in JavaScript (ES5; ES6 introduces let)

var a = 1;
function four() {
  if (true) {
    var a = 4;
  }
  alert(a); // alerts '4', not the global value of '1'
}

4. Intermediate: Object properties

var a = 1;
function five() {
  this.a = 5;
}
alert(new five().a); // alerts '5'

5. Advanced: Closure

var a = 1;
var six = (function() {
  var a = 6;
  return function() {
    // JavaScript "closure" means I have access to 'a' in here,
    // because it is defined in the function in which I was defined.
    alert(a); // alerts '6'
  };
})();

6. Advanced: Prototype-based scope resolution

var a = 1;
function seven() {
  this.a = 7;
}
// [object].prototype.property loses to
// [object].property in the lookup chain. For example...
// Won't get reached, because 'a' is set in the constructor above.
seven.prototype.a = -1;
// Will get reached, even though 'b' is NOT set in the constructor.
seven.prototype.b = 8;
alert(new seven().a); // alerts '7'
alert(new seven().b); // alerts '8'

7. Global+Local: An extra complex Case

var x = 5;
(function () {
    console.log(x);
    var x = 10;
    console.log(x);
})();
This will print out undefined and 10 rather than 5 and 10 since JavaScript always moves variable declarations (not initializations) to the top of the scope, making the code equivalent to:
var x = 5;
(function () {
    var x;
    console.log(x);
    x = 10;
    console.log(x);
})();

8. Catch clause-scoped variable

var e = 5;
console.log(e);
try {
    throw 6;
} catch (e) {
    console.log(e);
}
console.log(e);