Create another file, called MockSharePointClient.ts in the same folder where the web part is located.
import { ISPListItem } from "./ISPListItem"; export default class MockSharePointClient { private static _listItems: ISPListItem[] = [ { Id: 1, Title: "First list item" }, { Id: 2, Title: "Second list item" }, { Id: 3, Title: "Third list item" }, { Id: 4, Title: "Fourth list item" }, { Id: 5, Title: "Fifth list item" }, { Id: 6, Title: "Sixth list item" }, { Id: 7, Title: "Seventh list item" }, { Id: 8, Title: "Eight list item" }, { Id: 9, Title: "Ninth list item" }, ]; public static get(restUrl: string, options?: any) : Promise<ISPListItem[]> { return new Promise<ISPListItem[]>((resolve) => { resolve(MockSharePointClient._listItems...