es-discuss Digest, Vol 95, Issue 45

[Allen Wirfs-Brock]

On Jan 19, 2015, at 5:32 AM, Fabrício Matté wrote:

> Your second example may break if the constructor is called via `.call()`/`.apply()` or as a CallExpression : MemberExpression or if it has been `.bind()`ed. Although these may look like corner cases, a good transform should cover these cases, especially CallExpression : MemberExpression as it is very common in Node.js land to have constructors exported as properties of an exported object.

This style of `this` testing is what was necessary to distinguish [[call]] from [[construct]] under the old ES6 @@create design.  And the above edge cases were what complicated all of the built-in constructors (and required branding checks and initialization flags) to fully cover.

It was the complexity of correctly identifying those edge cases which lead us to recommending that JS code should never try to do it, and JS developers shouldn't try to defined classes whose constructors did an implicit 'new' when called. 

With 'new.target'  that complexity all goes away and I see no particularly reason to discourage constructors that 'new' themselves when called as functions.

Allen
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