Chaining custom commands and typescript

[reallymello]

I'm still new to typescript and I'm experimenting with converting existing nightwatch projects to using typescript. What is the best way/pattern to include the methods brought in through custom commands and have them work in chaining?

For example, in the below example I am trying to chain in .axeInject() and then .axeRun after calling assert which returns NightwatchAPI, but since it doesn't exist there (since JS + Nightwatch adds it in through the custom_commands functionality) it errors since TypeScript is more strict.
"Property 'axeInject' does not exist on type '(browser: ICustomBrowser) => void'.ts(2339)"

"Example a11y test": function (browser: ICustomBrowser) {
    browser
      .url("https://www.w3.org/WAI/demos/bad/after/home.html")
      .assert.title("Welcome to CityLights! [Accessible Home Page]")
      .axeInject()
      .axeRun("body", {
        rules: {},
      })
      .end();
 }

import {
  NightwatchAPI,
  NightwatchBrowser,
} from "nightwatch";

export interface ICustomBrowser extends NightwatchBrowser {
  testId: number;
}