typescript infer generics from properties

Question

I'm trying to create a class that merges two other classes that have one or more generics. The merged class needs to infer all of the generic types from the original two classes. I've attempted to use the infer keyword but I'm not sure I'm using it correctly or it just hasn't clicked for me. Also, all the examples I've seen only infer a single generic type parameter, but in my case, I need to infer multiple generics from a single type. Example in TS playground that is the structure I need, just missing the type inference to the FooBar properties:

class Foo<A, B> {
  constructor(public a: A, public b: B) {}
}

class Bar<C> {
  constructor(public c: C) {}
}

class FooBar <F extends Foo<any, any>, B extends Bar<any>> {

  // How do I infer these properties?
  a: any;
  b: any;
  c: any;

  constructor(foo: F, bar: B) {
    this.a = foo.a;
    this.b = foo.b;
    this.c = bar.c;
  }
}

const foo = new Foo(1, 'two');
foo.a // ts knows this is 'number'
foo.b // ts knows this is 'string'

const bar = new Bar(true);
bar.c // ts knows this is 'boolean'

const foobar = new FooBar(foo, bar);
foobar.a // this type is now 'any'
foobar.b // this type is now 'any'
foobar.c // this type is now 'any'

Oleksandr Kovalenko · Accepted Answer · 2021-07-22 16:21:09Z

1

class FooBar <F extends Foo<any, any>, B extends Bar<any>> {

  a: F['a'];
  b: F['b'];
  c: B['c'];

  constructor(foo: F, bar: B) {
    this.a = foo.a;
    this.b = foo.b;
    this.c = bar.c;
  }
}

answered Jul 22, 2021 at 16:21

Oleksandr Kovalenko

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2 Comments

iceblueorbitz Over a year ago

Thanks, I just realized this answers the question as asked - however it infers the class property not the generic directly which is what I'm looking for. The basic example I made allowed a simpler method of extracting the type - but in my real-world case I can't access the type off a property - it's just a generic that get's referenced as return values. Can this answer be updated to reflect getting the generic directly without accessing a property type?

iceblueorbitz Over a year ago

On second thought - this answer is very useful for the way I worded the question - I'll update the title and leave this and ask a new question.

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