Hi,

Thanks for this great library.

I'm developing a subclass by extending BigNumber to add my custom utility methods. However, when I tried to access superclass methods, I got type errors from TypeScript. (The code works as expected; I just got TypeScript typing errors).

class ExtendedNumber extends BigNumber {
  foo(): number {
    return 1000;
  }

  bar(x: number): this {
    const c = this.plus(x);
    const d = c.foo(); // <--------------- ERROR: Property 'foo' does not exist on type 'BigNumber'. ts(2339)
    return this;
  }
}

const value = new ExtendedNumber("10.1");
console.log(value.foo());
console.log(value.bar(1));

The problem is that the return type of methods is hard-coded as BigNumber. They should be this instead of BigNumber.

For example see the original code from bignumber.d.ts file below:

plus(n: BigNumber.Value, base?: number): BigNumber;

It should be:

plus(n: BigNumber.Value, base?: number): this;

Kind Regards,