Hardhat calculate bytes4 interface

helpers.ts

// Import contract factory generated from `hardhat compile` with typechain
import { IERC165__factory, IERC2981__factory } from '@custom-types/contracts'

import { ethers } from 'hardhat'
import { utils } from 'ethers'

const getInterfaceID = (contractInterface: utils.Interface) => {
    let interfaceID = ethers.constants.Zero;
    const functions: string[] = Object.keys(contractInterface.functions);
    for (let i = 0; i < functions.length; i++) {
        interfaceID = interfaceID.xor(contractInterface.getSighash(functions[i]));
    }
    return interfaceID;
}    
        
const ERC2981bytes = getInterfaceID(IERC165__factory.createInterface())
    .xor(getInterfaceID(IERC2981__factory.createInterface()))
    .toHexString()
        
// Use in tests (remember solidity overrides go from right to left)
describe('Supports interface', () => {
    it('does not support random interface', async () => {
        await expect(cats.supportsInterface('0x0')).to.be.reverted
    })
    
    it('does support ERC165', async () => {
        const supports = await cats.supportsInterface('0x01ffc9a7')
        expect(supports).to.be.true
    })

    it('does support ERC2981', async () => {
        const supports = await cats.supportsInterface(getERC2981InterfaceId())
        expect(supports).to.be.true
    })
})