What is the scope of variables in JavaScript?

I think about the best I can do is give you a bunch of examples to study. Javascript programmers are practically ranked by how well they understand scope. It can at times be quite counter-intuitive.

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;    alert(a); // alerts '3'  }

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'

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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);

   This will print out 5, 6, 5. Inside the catch clause e shadows global and local variables. But this special scope is only for the caught variable. If you write var f; inside the catch clause, then it's exactly the same as if you had defined it before or after the try-catch block.