Now, open your web part TypeScript file and insert the following import statements at the beginning section of the file before the web part class definition.

import { Environment, EnvironmentType } from '@microsoft/sp-core-library'; 
import { ISPListItem } from "./ISPListItem"; 
import MockSharePointClient from "./MockSharePointClient"; 

The first line imports Environment and EnvironmentType from sp-core-library; we use these to find out if we are running from a local Workbench and thus need to use mock data.

Inside the web part class definition, insert the following functions.

private _getMockListData(): Promise<ISPListItem[]> { 
  return MockSharePointClient.get("") 
    .then((data: ISPListItem[]) => { 
          return data; 
      }); 
} 
 
private _getListItems(): Promise<ISPListItem[]> { 
  if (Environment.type === EnvironmentType.Local) { 
    return this._getMockListData(); ...