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A constructor is just a function and failing to invoke a constructor with new leads to errors. Not syntax errors but logical errors. Following a naming convention can certainly help, but it doesn't enforce the correct behavior. If you invoke a constructor without new, then this will refer to the global object, called window in browsers.

Here's a pattern that helps you make sure your constructor always behaves as constructor. Instead of adding all members to this, you add them to that.

In this implementation that is the same as this unless this is window which means someone forgot to use new.

function Person() { 
  var that = (this === window) ? {} : this;
 
  that.name = name;
  that.say = function() {
    return "I am " + that.name;
  };
 
  return that; 
}

window is only in browsers

Another way, if you work in non-browser environment is to check if this is an instance of your Person constructor.

Or name-agnostic, general-purpose way is to use arguments.callee instead of hard-coding the constructor name.

this instanceof Person
this instanceof arguments.callee

Tags: constructors, Object creation patterns

This entry was posted on Monday, September 14th, 2009 and is filed under Object creation patterns. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.