What is the reason ES6 class constructors cant be called as normal functions?

2024/2/27 7:31:45

ES6 class constructors can't be called as normal functions. According to ES6 a TypeError should be raised when this is done. I used to think that classes were just syntactic sugar for a constructor function + functions in the prototype, but this makes it slightly not so.

I'm wondering, what was the rationale behind this? Unless I missed something, it prevents calling the function with a custom this, which could be desirable for some patterns.

Answer

A revisit to the ES6 spec shows how calling a Class function object without new is disabled by combining sections 9.2.9 and 9.2.1:

9.2.9 MakClassConstructor (F)
...
3. Set F’s [[FunctionKind]] internal slot to "classConstructor".

and when specifying the [[call]] method as opposed to the [[contruct]] method of a function:

(9.2.1) 2. If F’s [[FunctionKind]] internal slot is "classConstructor", throw a TypeError exception.

No restrictions are placed on calling function in section "11.2.3 "Function calls" of ES5.1.

So you are not missing anything: you can't use apply on a class constructor function.

The major rationale is probably both to make class extensions a fairly rigorous exercise, and to detect some early forms of error. For example you can't call Promise except as a constructor - and leaving out new before a call to Promise is a programming error. In regards extending classes, note that the constructor property of class instances is correctly set (the last class after possibly multiple extensions) and the the .prototype property of the class constructor is read only - you can't dynamically change the prototype object used to construct class instances, even though you could change the prototype property of a constructor function.

I used to think classes were syntactic sugar but have moved away from the concept.

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