How does interfaces with construct signatures work?

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Construct signatures in interfaces are not implementable in classes; they’re only for defining existing JS APIs that define a ‘new’-able function. Here’s an example involving interfaces new signatures that does work:

interface ComesFromString {
    name: string;
}

interface StringConstructable {
    new(n: string): ComesFromString;
}

class MadeFromString implements ComesFromString {
    constructor (public name: string) {
        console.log('ctor invoked');
    }
}

function makeObj(n: StringConstructable) {
    return new n('hello!');
}

console.log(makeObj(MadeFromString).name);

This creates an actual constraint for what you can invoke makeObj with:

class Other implements ComesFromString {
    constructor (public name: string, count: number) {
    }
}

makeObj(Other); // Error! Other's constructor doesn't match StringConstructable

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