TypeScript: Intersection Types for construct signature and a type doesn't work, but for call signature and a type does

Question

I would like to create an intersection type, which consists of a constructor signature and some other type.

type CustomMixer<T1, T2> = (new (...args: any[]) => T1) & T2

Generelly I need this to annotate the return type of a function, which sets some static methods to a Class(Function), like Object.assign(MyCtorFn, staticMethods)

TypeScript allows to create such types, return it, but I can't then initialize the type with the new keyword.

const MyType: CustomMixer<Foo, Bar> = someFactoryFunction()
const x = new MyType()

Cannot use 'new' with an expression whose type lacks a call or construct signature.

But, if CustomMixer intersection consists of a call signature and other type, then everything works as expected.

type CustomMixer<T1, T2> = ((...args: any[]) => T1) & T2
const MyType: CustomMixer<Foo, Bar> = someFactoryFunction()
MyType.| // Autocompletion for T2 works
const x = MyType()
x.|// Autocompletion for T1 works

Here is the link to TS Playground

Is it possible to make the first example work? Thank you.

Nitzan Tomer · Accepted Answer · 2017-07-12 13:45:33Z

1

You can do this:

type CustomMixer<T1, T2> = T1 & T2;

function CustomMixerFactory<T1, T2>(): CustomMixer<T1, T2> {
    // Mimic typed return
    return null as CustomMixer<T1, T2>;
}

const MyType = CustomMixerFactory<typeof Foo, Baz>();

MyType.isBaz(); // fine
let myType = new MyType(); // fine
myType.isFoo(); // fine

answered Jul 12, 2017 at 13:45

Nitzan Tomer

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1 Comment

tenbits Over a year ago

Thank you, with the solution I even better undestand the type system in TypeScript.

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