ECMA-262 states that a constructor is an object that supports the [[Construct]] internal method. I suspect that BigInt and Symbol support [[Construct]] because they are included in Constructor Properties of the Global Object . However, both return a TypeError when called with the new operator: try {…

From the point of view of the spec, BigInt and Symbol are constructors, and have a [[Construct]] internal method, but its behavior is to return a TypeError. So for (most?) practical purposes, they look like not-constructors.

[image] MaineGardener: Why aren't BigInt and Symbol just function objects that […] do not have a seemingly unnecessary prototype property? The .prototype is not unnecessary, it is used to supply the methods and getters on BigInt and Symbol objects.

Another way to look at it is that they are private constructors. const i = Object(1n); i instanceof BigInt; true If #constructor became part of the language perhaps BigInt and Symbol would be modelled that way.

Like all objects, BigInt and Symbol have a [[Prototype]] internal slot which points to a parent object from which the objects inherit properties. [[Prototype]] is different than a prototype property which constructors have. A constructor’s prototype property points to the object that will become the…

As per Constructor definition - #3 by bergus and Constructor definition - #4 by aclaymore , while new Symbol() doesn't create an instance of Symbol. These instances do exist and the .prototype is used: const s = Symbol("abc"); Object.getPrototypeOf(s) === Symbol.prototype; Reflect.ownKeys(Symbol.pr…

Yes, were talking about .prototype here. Those objects are still getting used as the [[prototype]] for instances that are getting created temporarily when accessing a property on primitives, such as Symbol("example").description or 1n.toString(). You can also explicitly construct them using Object(S…

[image] MaineGardener: Note that the error includes is not a constructor The spec only defines the type of Error that is thrown, the error message is up to the implementation. It just so happens that many implementations have mutually chosen the same message of "is not a constructor".

Thanks bergus, aclaymore, and jmdyck. Very helpful. It's interesting that the Chrome and Node.js implementations (and surely others) interpret the TypeError as "is not a constructor". It suggests to me that (perhaps) the implementation teams do not view Symbol and BigInt as constructors. I would gue…

[image] MaineGardener: We know, of course, that the spec defines a constructor as a function object that supports [[Construct]]. But, I don't know an empirical way to test this Proxies can be used to check for the [[Construct]] slot, because they will only call the 'construct' hook if the tar…

Constructor definition

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bergus February 11, 2024, 4:08am 3

The .prototype is not unnecessary, it is used to supply the methods and getters on BigInt and Symbol objects.

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Constructor definition

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