+2  A: 

To concisely answer your question, everything beneath the LeftHandSideExpression production is a valid LeftHandSideExpression.


I think the question you are really asking is:

What is a valid LeftHandSideExpression and also assignable?

The answer to that is anything that resolves to a Reference which is a well defined concept in the specification. In your example

new Object = 1;

The new Object is a valid LeftHandSideExpression but it does not resolve to a Reference.

(new Object).x = 1;

The left hand side is a MemberExpression . IdentifierName which according to the spec the final step is:

Return a value of type Reference ...


If you consider it 2 separate properties it makes a lot more sense.

  1. Is it a valid LeftHandSideExpression?
  2. Is it a valid reference?

Property 1 is determined in the syntactical analysis phase and property 2 is determined in the semantic analysis phase. Check out 8.7.2 PutValue (V, W) for more details.

Here is a full explanation in the specification itself:

8.7 The Reference Specification Type

The Reference type is used to explain the behaviour of such operators as delete, typeof, and the assignment operators. For example, the left-hand operand of an assignment is expected to produce a reference. The behaviour of assignment could, instead, be explained entirely in terms of a case analysis on the syntactic form of the left-hand operand of an assignment operator, but for one difficulty: function calls are permitted to return references. This possibility is admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a reference and there is no provision for a user-defined function to return a reference. (Another reason not to use a syntactic case analysis is that it would be lengthy and awkward, affecting many parts of the specification.)


After taking a look at your suggestion I believe it would throw off certain valid expressions (Note: I don't condone this.)

function OuterObj() {
    this.Name = "Outer";
    this.InnerObj = function() {
        this.Name = "Inner";
    }
}

var obj; (obj = new new OuterObj().InnerObj).Name = "Assigned";

This is a case where NewExpression is important

ChaosPandion
True. I always thought that phase 2 is carried out at run-time while phase 1 occurs at parse-time. Thus, `if (false) { new Object = 1; }` should not be a syntax error, and does ideed work in Safari. A look at the Firefox source code led me to [this](https://bugzilla.mozilla.org/show_bug.cgi?id=509098#c11): Firefox treats this one as a special case. (Fun fact: error code is called `JSMSG_BAD_LEFTSIDE_OF_ASS`)
Pumbaa80
@Pumbaa80 - I think doing it this way really makes sense in a dynamic language.
ChaosPandion