Javascript Autocast

A typical day at work requires me to constantly switch between writing code in C# and Javascript. This effects the way I write code in both languages. Since C# is more strict that Javascript, my coding style leans towards C# and sometimes I find myself writing Javascript code in C# style, which can be viewed as good or a bad thing.

Every language has its goodies that if we take advantage of can produce better quality code.

Let's take a common task as example. We want to test a string variable for null and empty.

In C#,I will do like:

string str;
...
if (str != null && !str.Equals(string.Empty)) { }

Applying the same C# style to Javascript will result in the following code:

var str;
...
if (str != null && str != '') { }

The above code is not optimized. In Javascript, we can utilitize 'autocast' feature that will reduce the code into:

var str;
...
if (str) { }

A null or an empty string autocasts into false, and therefore the condition is evaluated as false.

As you can see, the code is much shorter and cleaner. Size does matter in Javascript. Shorter code translates into faster download time.

Autocast also applies to other types/conditions:

• Undefined variable is evaluated as false. A variable is undefined if it hasn't been assigned a value.
  Example:
  
  var myVar;
  if (myVar) { } // false
  
  
• Empty string is evaluated as false
  
  var myStr = '';
  if (myStr) { } // false
  
  var myStr2 = 'ABC';
  if (myStr2) { } // true
  
  
• Zero is evaluated as false
  
  var myNum = 0;
  if (myNum) { } // false
  
  var myNum2 = 1;
  if (myNum2) { } // true
  
  
• Null is evaluated as false
  
  var myObj = null;
  if (myObj) { } // false
  
  
• Object is evaluated as true
  
  var myObj = {};
  var myObj2 = new Object();
  if (myObj) { } // true
  if (myObj2) { } // also true
  
  
• Empty array is evaluated as true
  An array is essentially an object, therefore it is evaluated as true.
  
  var myArray = [];
  var myArray2 = new Array();
  if (myArray) { } // true
  if (myArray2) { } // true