Constructor behaving differently using ES6 shorthand notation

The specification isn't very direct about explaining this, but we can follow a short chain..

We'll start at EvaluateNew, since that's the behaviour we're wondering about. Step 7 is clearly the one we're looking for here:

7. If IsConstructor(constructor) is false, throw a TypeError exception.

So IsConstructor is where we need to look next.

Both the summary and the steps describe this:

The abstract operation IsConstructor determines if argument, which must be an ECMAScript language value, is a function object with a [[Construct]] internal method.

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1. If Type(argument) is not Object, return false.
2. If argument has a [[Construct]] internal method, return true.
3. Return false.

So, judging by the looks of it, our obj1.a doesn't have a [[Construct]] internal method. Let's look for where it says that it shouldn't have one..

Here's what we're looking for, PropertyDefinitionEvaluation. The first step is useful here:

Let methodDef be DefineMethod of MethodDefinition with argument object.

That calls DefineMethod with just one argument, object. Let's look at DefineMethod - here's what we need:

With parameters object and optional parameter functionPrototype.

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6. If functionPrototype was passed as a parameter, let kind be Normal; otherwise let kind be Method.
7. Let closure be FunctionCreate(kind, [more arguments snipped]).

Since functionPrototype was not passed as a parameter, the kind is Method. Let's look at what FunctionCreate does with that:

2. If kind is not Normal, let allocKind be "non-constructor".
3. Else, let allocKind be "normal".
4. Let F be FunctionAllocate([other arguments snipped], allocKind).

Now we're getting close! We just need to look at FunctionAllocate does with allocKind (which is "non-constructor" as per the above steps), which is what gives a function all of its internal methods and such.

3. If functionKind is "normal", let needsConstruct be true.
4. Else, let needsConstruct be false.

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6. Let F be a newly created ECMAScript function object with the internal slots listed in Table 27. All of those internal slots are initialized to undefined.

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9. If needsConstruct is true, then

   a. Set F's [[Construct]] internal method to the definition specified in 9.2.2.

   b. Set the [[ConstructorKind]] internal slot of F to "base".

Finally! If we go through the relevant steps, we can see since functionKind isn't "normal", needsConstruct becomes false, and so a [[Construct]] internal method is never assigned! Then IsConstructor sees that and returns false, and so EvaluateNew fails.

MDN describes this behaviour very simply:

All method definitions are not constructors and will throw a TypeError if you try to instantiate them.

..but now you know how they aren't constructors, officially.

Seems the original discussion happens here: https://github.com/rwaldron/tc39-notes/blob/master/es6/2012-07/july-26.md

MM: three reasons for this:

• precedent in builtins
• using a method as a constructor is generally nonsense
• to freeze a class, I have to freeze the .prototype of the methods on the prototype!!

and

AWB: suggestion: concise methods should be the same for both classes and object literals

• strictness
• enumerability
• constructability
• attributes

That's how both class methods and object methods became non-constructable

Methods declared using this syntax are not intended to be constructable

Reference here