JavaScript Scoping by Example

Understanding the scoping rules of JavaScript is one of the fundamental things to learn when getting started with the language. I just found this link posted by @codylindley on Twitter. It goes over some basic principles of jQuery, however the first part covers native JavaScript scoping. It is an interesting example to get an understanding about it.

Take the following piece of JavaScript:

var x = "Hi";

var someFunction = (function(){
  document.getElementById("out").innerHTML = x;
});

someFunction();

The example is quite simple. There is x, which is defined in the global scope, followed by the definition of a function which gets stored in the someFunction variable. someFunction gets called immediately afterwards, printing out the value of x to the HTML element "out". When executed, we get "Hi" as a result.
Why does this happen? In order to understand this, you need to understand that JavaScript has function scope, meaning that when printing out x, it will first search for a definition within the function itself, and then it will walk up the prototype chain in order to find a corresponding variable definition. In the specific example above, it will go up till it reaches the global (window) object where x is defined.

If now the example above gets modified as follows, can you guess what will be its output??

var x = "Hi";

var someFunction = (function(){
  document.getElementById("out").innerHTML = x;
  var x = "Hi, there!";
});

someFunction();

Actually the output is "undefined". This is due to the lexical function scoping, meaning the scoping rules are applied at function-definition, not at runtime. Btw, this kind of mechanism also allows for having closures. Hence, x is defined within the function's scope, but just not at the moment when it was printed out.

To fix this one would have to explicitly let x point to the globally defined one like

var x = "Hi";

var someFunction = (function(){
  document.getElementById("out").innerHTML = window.x;
  var x = "Hi, there!";
});

someFunction();

Now it again works.