projects/sit-web-cache/src/app.service.ts

import { Injectable, HttpStatus } from '@nestjs/common';
import * as fs from 'fs-extra';

import { Response } from 'express';
@Injectable()
export class AppService {
  saveGridFailures(sits: any, res: Function) {
    const jsonString = JSON.stringify(sits);
    return fs.writeFile('./store/public-sits.json', jsonString, err => {
      if (err) {
        throw err;
      } else {
        res(true, 'Successfully wrote file');
      }
    });
  }

  getSITs(res: Function) {
    fs.readFile('./store/public-sits.json', 'utf8', function(err, data) {
      if (err) throw err;
      else res(data); //do operation on data that generates say resultArray;
    });
  }
}