Can x.y and x['y'] ever result in different behavior for Host Objects?

Question

For a normal ("Native") JavaScript objects, where the property name is a valid identifier, the expressions x.y and x["y"] are always semantically equivalent.

Is this equivalency guaranteed for Host Objects?

[..an] object supplied by the host environment to complete the execution environment of ECMAScript.

If not, are there notable exceptions? (In Cocoon, Crosswalk, IE, etc?)

In particular I am interested in the Window (window) and other DOM Objects; and if there are any known "exceptions" to the expected behavior, or if there are any environments in where such is possible.

Answer 1

It should be identical. From Section 11.2.1 (Property Accessors) of the Ecma-262 Edition 5.1, we have:

Properties are accessed by name, using either the dot notation:

MemberExpression . IdentifierName 
CallExpression . IdentifierName

or the bracket notation:

MemberExpression [ Expression ]
CallExpression [ Expression ]

The dot notation is explained by the following syntactic conversion:

MemberExpression . IdentifierName

is identical in its behaviour to

MemberExpression [ <identifier-name-string> ]

and similarly

CallExpression . IdentifierName

is identical in its behaviour to

CallExpression [ <identifier-name-string> ]