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    <p>To concisely answer your question, everything beneath the <code>LeftHandSideExpression</code> production is a valid <code>LeftHandSideExpression</code>.</p> <hr> <p>I think the question you are really asking is:</p> <blockquote> <p>What is a valid <code>LeftHandSideExpression</code> and also assignable?</p> </blockquote> <p>The answer to that is anything that resolves to a <code>Reference</code> which is a well defined concept in the specification. In your example</p> <pre><code>new Object = 1; </code></pre> <p>The <code>new Object</code> is a valid <code>LeftHandSideExpression</code> but it does not resolve to a <code>Reference</code>.</p> <pre><code>(new Object).x = 1; </code></pre> <p>The left hand side is a <code>MemberExpression . IdentifierName</code> which according to the spec the final step is:</p> <blockquote> <p>Return a value of type Reference <strong>...</strong></p> </blockquote> <hr> <p>If you consider it 2 separate properties it makes a lot more sense.</p> <ol> <li>Is it a valid LeftHandSideExpression?</li> <li>Is it a valid reference? </li> </ol> <p>Property 1 is determined in the syntactical analysis phase and property 2 is determined in the semantic analysis phase. Check out <strong>8.7.2 PutValue (V, W)</strong> for more details.</p> <p>Here is a full explanation in the specification itself:</p> <p><strong>8.7 The Reference Specification Type</strong></p> <p>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.)</p> <hr> <p>After taking a look at your suggestion I believe it would throw off certain valid expressions <em>(Note: I don't condone this.)</em></p> <pre><code>function OuterObj() { this.Name = "Outer"; this.InnerObj = function() { this.Name = "Inner"; } } var obj; (obj = new new OuterObj().InnerObj).Name = "Assigned"; </code></pre> <p>This is a case where <code>NewExpression</code> is important</p>
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    1. COTrue. 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`)
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    2. CO@Pumbaa80 - I think doing it this way really makes sense in a dynamic language.
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