Dynamic constructor name in js

Question

I want to do exactly the same as this question but use the function as a constructor.

> function nameFunction(name, body) {
   return {[name](...args) {return body(...args)}}[name]
}

> b = nameFunction('c', function () {})
[Function: c]
> typeof b
'function'

b is a function, yet it is not a constructor:

> new b()
Uncaught TypeError: b is not a constructor

If done in the standard way, it is a constructor:

> b = function() {}
[Function: b]
> new b()
b {}

But the name is b, not c.

How can I build a constructor with the given name and not using eval?

`nameFunction` returns an object, not a function? If you would like to use `new` it should return a class or a "regular" function (not an arrow function). — evolutionxbox, Mar 16 '21 at 20:44
@evolutionxbox how is b in the code given an arrow function? How do you show that it is an object? `typeof b === 'function'` — Gabriel Furstenheim, Mar 16 '21 at 20:48
You're right. It's not an arrow function (I didn't say it was). My recommendation is to return a class or regular function. — evolutionxbox, Mar 16 '21 at 20:49
@evolutionxbox but I want the name of the created object to be dynamic — Gabriel Furstenheim, Mar 16 '21 at 20:50
I couldn't really figure out why the function being returned wasn't a constructor function. It is most likely be due to the fact it's being made inside an object. — evolutionxbox, Mar 16 '21 at 21:03
[This answer](https://stackoverflow.com/a/47350988/989920) may be your best option? — evolutionxbox, Mar 16 '21 at 21:12

score 2 · Accepted Answer · answered Mar 18 '21 at 02:37

The answer provided by georg is correct but incomplete. In order to use the dynamically named function as a constructor, not only do you need to use an ordinary function but also you need to preserve the this context. We can do this by applying this and arguments to the body as follows.

function nameFunction(name, body) {
    const object = {
        [name]: function () {
            return body.apply(this, arguments);
        }
    };

    return object[name];
}

const Bar = nameFunction("Foo", function (foo) {
    this.foo = foo;
});

console.log(Bar.name, new Bar(42));

score 1 · Answer 2 · answered Mar 16 '21 at 21:15

1

Methods are not constructors (see https://262.ecma-international.org/7.0/#sec-functioncreate and https://262.ecma-international.org/7.0/#sec-functionallocate). Just define it as an ordinary function:

function nameFunction(name, body) {
   return {[name]: function(...args) {return body(...args)}}[name]
}

b = nameFunction('c', function () {})

new b

answered Mar 16 '21 at 21:15

georg

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I'm not sure this entirely works? `b = nameFunction('c', function () { this.x = 'x' }); new b() // Object { } <- no x property`? – evolutionxbox Mar 16 '21 at 21:17

Dynamic constructor name in js

2 Answers2