A few months ago, I asked a group of web developers what the new operator does in Javascript. There wasn't much of a response, but I did eventually figure it out. Sometimes you just have to wade through the specifications. Here's the outcome of my research:

[ECMA Version 3 (1999), section 11.2.2, page 56]

The production NewExpression : new NewExpression is evaluated as follows:

  1. Evaluate NewExpression.
  2. Call GetValue(Result(1)).
  3. If Type(Result(2)) is not Object, throw a TypeError exception.
  4. If Result(2) does not implement the internal [[Construct]] method, throw a TypeError exception.
  5. Call the [[Construct]] method on Result(2), providing no arguments (that is, an empty list of arguments).
  6. Return Result(5).

[Section 13.2.2, page 86]

When the [[Construct]] property for a Function object F is called, the following steps are taken:

  1. Create a new native ECMAScript object.
  2. Set the [[Class]] property of Result(1) to "Object".
  3. Get the value of the prototype property of the F.
  4. If Result(3) is an object, set the [[Prototype]] property of Result(1) to Result(3).
  5. If Result(3) is not an object, set the [[Prototype]] property of Result(1) to the original Object prototype object as described in
  6. Invoke the [[Call]] property of F, providing Result(1) as the this value and providing the argument list passed into [[Construct]] as the argument values.
  7. If Type(Result(6)) is Object then return Result(6).
  8. Return Result(1).

I just wanted to put that somewhere where it wouldn't get lost.

It also took me a while to realize that Result(N) means "the product after performing the operation of step N.

The new keyword allows you to define objects with prototypes, so that you can have a class heirarchy that doesn't require external maintenence. I strongly suspect that Doug Crockford's lecture on "prototypal inheritance" would be edifying here.