JavaScript borrowed the new and delete keywords from its less-dynamic predecessor languages. They feel a bit out of place in a garbage collected language and are a source of confusion for newbies – one of the reasons popular libraries such as jQuery, d3, and Ember.js have adopted APIs that don’t require using new at all. In this post I’ll show you one way do it, and why you should consider it for your next JavaScript library.

In C++, new and delete are symmetric operators that combine memory management and object lifecycle operations. new allocates an instance and calls the constructor, and delete calls the destructor and deallocates it. In garbage collected languages, delete isn’t necessary; Java doesn’t have it, for example. It’s one of JavaScript’s quirks that it does use delete , and for a purpose (removing a property from an object) that’s not symmetric with new . You’ll sometimes see confused JavaScript programmers trying to delete plain objects.

new can itself be a source of confusion. Looking at instantiation with new , it’s easy to see why: new User() requires both a language keyword and a unique syntactic form. In other scripting languages, neither are needed: instantiation is done via regular function notation, either via a class method as in Ruby ( User.new ), or calling the class object as a function as in Python ( User() ).

What’s worse, in JavaScript, forgetting new when calling a constructor can produce some very strange behavior – leaving variables undefined and polluting the global scope. John Resig gave a great rundown of the issues and proposed a solution, often dubbed the ‘instanceof trick’:

The instanceof trick 1 2 3 4 5 6 7 8 9 function User ( first , last ) { if ( ! ( this instanceof User )) return new User ( first , last ); this . first = first ; this . last = last ; } User . prototype . fullName = function () { return this . first + ' ' + this . last ; }

Class constructors defined with the instanceof trick can be called with or without new :

1 2 3 4 5 var userViaNew = new User ( "John" , "Firebaugh" ); userViaNew . fullName (); // "John Firebaugh" var userDirect = User ( "John" , "Resig" ); userDirect . fullName (); // "John Resig"

In either case, the result is a newly allocated and initialized User.

John Resig goes on to show how to create a reusable function that builds constructors that use the instanceof trick. This way, instead of writing it out manually for every class, you write it once and use a higher-level function to declare classes and their prototype properties:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 // The `Class.extend` function defines a new constructor that uses // the instanceof trick internally, and then calls `initialize`: var User = Class . extend ({ initialize : function ( first , last ) { this . first = first ; this . last = last ; }, fullName : function () { return this . first + ' ' + this . last ; } }); var userViaNew = new User ( "John" , "Firebaugh" ); var userDirect = User ( "John" , "Resig" );

APIs that use the instanceof trick internally often publicly document only the newless form of instantiation. This minimizes the API surface area, gives new users less to learn, and avoids the awkward aspects of new . Another advantage is that it allows the implementation to choose either the Module pattern or classic prototypal instantiation without changing the public API. For example, we can rewrite the User class to use the Module pattern without changing how it’s used:

1 2 3 4 5 6 7 8 9 10 function User ( first , last ) { function fullName () { return first + ' ' + last ; } return { first : first , last : last , fullName : fullName }; }

Similarly, if we had started out with the Module pattern, but discovered that we needed the memory efficiency of prototypal instantation, we could switch without changing client code.

An alternative that some library use is to pair each constructor with a factory function of the same name but with leading lower-case, e.g. User and user :

1 2 3 4 5 6 function user ( first , last ) { return new User ( first , last ); } var userViaNew = new User ( "John" , "Firebaugh" ); var userViaFactory = user ( "John" , "Resig" );

This works, but again, it increases the size of the API you need to document and users need to learn, and unlike the instanceof trick, there’s no way to implement it in a reusable form. You’re stuck writing a factory wrapper for every class.

A newless API works great for as jQuery and d3, and I’ve found it very useful in iD as well. Consider using it for your next library.