[ECMA Version 3 (1999), section 11.2.2, page 56]
The production NewExpression : new NewExpression is evaluated as follows:
- Evaluate NewExpression.
- Call GetValue(Result(1)).
- If Type(Result(2)) is not Object, throw a TypeError exception.
- If Result(2) does not implement the internal [[Construct]] method, throw a TypeError exception.
- Call the [[Construct]] method on Result(2), providing no arguments (that is, an empty list of arguments).
- 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:
- Create a new native ECMAScript object.
- Set the [[Class]] property of Result(1) to "Object".
- Get the value of the prototype property of the F.
- If Result(3) is an object, set the [[Prototype]] property of Result(1) to Result(3).
- If Result(3) is not an object, set the [[Prototype]] property of Result(1) to the original Object prototype object as described in 18.104.22.168.
- 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.
- If Type(Result(6)) is Object then return Result(6).
- 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.