Is JavaScript’s “new” keyword considered harmful? [closed]

Is JavaScript’s “new” keyword considered harmful? [closed]

In another question, a user pointed out that the new keyword was dangerous to use and proposed a solution to object creation that did not use new. I didn’t believe that was true, mostly because I’ve used Prototype, Scriptaculous and other excellent JavaScript libraries, and everyone of them used the new keyword.
In spite of that, yesterday I was watching Douglas Crockford’s talk at YUI theater and he said the exactly same thing, that he didn’t use the new keyword anymore in his code (Crockford on JavaScript – Act III: Function the Ultimate – 50:23 minutes).
Is it ‘bad’ to use the new keyword? What are the advantages and disadvantages of using it?


Solution 1:

Crockford has done a lot to popularize good JavaScript techniques. His opinionated stance on key elements of the language have sparked many useful discussions. That said, there are far too many people that take each proclamation of “bad” or “harmful” as gospel, refusing to look beyond one man’s opinion. It can be a bit frustrating at times.

Use of the functionality provided by the new keyword has several advantages over building each object from scratch:

  1. Prototype inheritance. While often looked at with a mix of suspicion and derision by those accustomed to class-based OO languages, JavaScript’s native inheritance technique is a simple and surprisingly effective means of code re-use. And the new keyword is the canonical (and only available cross-platform) means of using it.
  2. Performance. This is a side-effect of #1: if I want to add 10 methods to every object I create, I could just write a creation function that manually assigns each method to each new object… Or, I could assign them to the creation function’s prototype and use new to stamp out new objects. Not only is this faster (no code needed for each and every method on the prototype), it avoids ballooning each object with separate properties for each method. On slower machines (or especially, slower JS interpreters) when many objects are being created this can mean a significant savings in time and memory.

And yes, new has one crucial disadvantage, ably described by other answers: if you forget to use it, your code will break without warning. Fortunately, that disadvantage is easily mitigated – simply add a bit of code to the function itself:

function foo()
   // if user accidentally omits the new keyword, this will 
   // silently correct the problem...
   if ( !(this instanceof foo) )
      return new foo();

   // constructor logic follows...

Now you can have the advantages of new without having to worry about problems caused by accidentally misuse. You could even add an assertion to the check if the thought of broken code silently working bothers you. Or, as some commented, use the check to introduce a runtime exception:

if ( !(this instanceof arguments.callee) ) 
   throw new Error("Constructor called as a function");

(Note that this snippet is able to avoid hard-coding the constructor function name, as unlike the previous example it has no need to actually instantiate the object – therefore, it can be copied into each target function without modification.)

John Resig goes into detail on this technique in his Simple “Class” Instantiation post, as well as including a means of building this behavior into your “classes” by default. Definitely worth a read… as is his upcoming book, Secrets of the JavaScript Ninja, which finds hidden gold in this and many other “harmful” features of the JavaScript language (the chapter on with is especially enlightening for those of us who initially dismissed this much-maligned feature as a gimmick).

Solution 2:

I have just read some parts of his Crockfords book “Javascript: The Good Parts”. I get the feeling that he considers everything that ever has bitten him as harmful:

About switch fall through:

I never allow switch cases to fall
through to the next case. I once found
a bug in my code caused by an
unintended fall through immediately
after having made a vigorous speech
about why fall through was sometimes
useful. (page 97, ISBN

About ++ and —

The ++ (increment) and — (decrement)
operators have been known to
contribute to bad code by encouraging
exessive trickiness. They are second
only to faulty architecture in
enabling viruses and other security
menaces. (page 122)

About new:

If you forget to include the new
prefix when calling a constructor
function, then this will not be
bound to the new object. Sadly, this
will be bound to the global object, so
instead of augmenting your new object,
you will be clobbering global
variables. That is really bad. There
is no compile warning, and there is no
runtime warning. (page 49)

There are more, but I hope you get the picture.

My answer to your question: No, it’s not harmful. but if you forget to use it when you should you could have some problems. If you are developing in a good environment you notice that.


About a year after this answer was written the 5th edition of ECMAScript was released, with support for strict mode. In strict mode, this is no longer bound to the global object but to undefined.

Solution 3:

Javascript being dynamic language there a zillion ways to mess up where another language would stop you.

Avoiding a fundamental language feature such as new on the basis that you might mess up is a bit like removing your shiny new shoes before walking through a minefield just in case you might get your shoes muddy.

I use a convention where function names begin with a lower case letter and ‘functions’ that are actually class definitions begin with a upper case letter. The result is a really quite compelling visual clue that the ‘syntax’ is wrong:-

var o = MyClass();  // this is clearly wrong.

On top of this good naming habits help. After all functions do things and therefore there should be a verb in its name whereas classes represent objects and are nouns and adjectives with no verb.

var o = chair() // Executing chair is daft.
var o = createChair() // makes sense.

Its interesting how SO’s syntax colouring has interpretted the code above.

Solution 4:

I am newbie to Javascript so maybe I am just not too experienced in providing a good view point to this. Yet I want to share my view on this “new” thing.

I have come from the C# world where using the keyword “new” is so natural that it is the factory design pattern that looks weird to me.

When I first code in Javascript, I don’t realize that there is the “new” keyword and code like the one in YUI pattern and it doesn’t take me long to run into disaster. I lose track of what a particular line is supposed to be doing when looking back the code I’ve written. More chaotic is that my mind can’t really transit between object instances boundaries when I am “dry-running” the code.

Then, I found the “new” keyword which to me, it “separate” things. With the new keyword, it creates things. Without the new keyword, I know I won’t confuse it with creating things unless the function I am invoking gives me strong clues of that.

For instance, with var bar=foo(); I have no clues as what bar could possibly be…. Is it a return value or is it a newly created object? But with var bar = new foo(); I know for sure bar is an object.

Solution 5:

Another case for new is what I call Pooh Coding. Winnie the Pooh follows his tummy. I say go with the language you are using, not against it.

Chances are that the maintainers of the language will optimize the language for the idioms they try to encourage. If they put a new keyword into the language they probably think it makes sense to be clear when creating a new instance.

Code written following the language’s intentions will increase in efficiency with each release. And code avoiding the key constructs of the language will suffer with time.

EDIT: And this goes well beyond performance. I can’t count the times I’ve heard (or said) “why the hell did they do that?” when finding strange looking code. It often turns out that at the time when the code was written there was some “good” reason for it. Following the Tao of the language is your best insurance for not having your code ridiculed some years from now.

Solution 6:

I wrote a post on how to mitigate the problem of calling a constructor without the new keyword.
It’s mostly didactic, but it shows how you can create constructors that work with or without new and doesn’t require you to add boilerplate code to test this in every constructor.

Here’s the gist of the technique:

 * Wraps the passed in constructor so it works with
 * or without the new keyword
 * @param {Function} realCtor The constructor function.
 *    Note that this is going to be wrapped
 *    and should not be used directly 
function ctor(realCtor){
  // This is going to be the actual constructor
  return function wrapperCtor(){
    var obj; // object that will be created
    if (this instanceof wrapperCtor) {
      // Called with new
      obj = this;
    } else {
      // Called without new. Create an empty object of the
      // correct type without running that constructor
      surrogateCtor.prototype = wrapperCtor.prototype;
      obj = new surrogateCtor();
    // Call the real constructor function
    realCtor.apply(obj, arguments);
    return obj;

  function surrogateCtor() {}

Here’s how to use it:

// Create our point constructor
Point = ctor(function(x,y){
  this.x = x;
  this.y = y;

// This is good
var pt = new Point(20,30);
// This is OK also
var pt2 = Point(20,30);