I am trying to add a testcase to our app test suite.
it requires changing two of our files before the test and
restoring them after the test

so we have a helper, which provides global reusable functions.

start our app
get the web content from our app (we only have 1 page)

etc etc..

so I added two functions to be called before and after the test begins

but, altho the code works perfectly under nodejs,
under jest the files are not written.

const os = require("node:os");
const fs = require("node:fs");
const jsdom = require("jsdom");

const indexFile = __dirname +"/../../../index.html";
const cssFile = __dirname + "/../../../css/custom.css";
const sampleCss = [
    ".region.row3 {",
    " top: 0;",
    "}",
    ".region.row3.left {",
    " top: 100%;"
    ,"}"
    ];
var indexData = [];
var cssData = [];

exports.startApplication = async (configFilename, exec) => {
    jest.resetModules();
    if (global.app) {
        await this.stopApplication();
    }
    // Set config sample for use in test
    if (configFilename === "") {
        process.env.MM_CONFIG_FILE = "config/config.js";
    } else {
        process.env.MM_CONFIG_FILE = configFilename;
    }
    if (exec) exec;
    global.app = require("../../../js/app");

    return global.app.start();
};

exports.fixupIndex =  () => {

    indexData = fs.readFileSync(indexFile).toString();
    let workIndexLines = indexData.split(os.EOL);
    for (let l in workIndexLines) {
        if (workIndexLines[l].includes("region top right")) {
            console.log("found it")
            workIndexLines.splice(l, 0, "      <div class=\"region row3 left\"><div class=\"container\"></div></div>");
            break;
        }
    }
    fs.writeFileSync(indexFile, workIndexLines.join(os.EOL), { flush: true });  
    cssData = fs.readFileSync(cssFile).toString();
    console.log("css ="+cssData)
    fs.writeFileSync(cssFile, sampleCss.join(os.EOL), { flush: true });
};

the actual testcase reads the content of the row3_left element
(this is a user extending our page template, and me trying to detect when they did that for other functions in our code that validate our template layout)

if I run
node tests/e2e/custom_region.js
it correctly modifies the files. so the paths are correct, the fs functions are correct and the file modifications are correct.
(I have a restore function which writes out the content read before the modify)

the testcase shown below calls

helper.fixupIndex()
helper.startApplication()

so the files are not used until the startApplication() executes

I don't see any restrictions listed in the jest doc about local file access..
and all the testcase I see are actually USING the file contents in expect() or some other validation operation.. not as part of setup

we are executing these tests as part of a github workflow on pull request merge.. so I don't want to make a permanently modified copy of the files
testcase

const helpers = require("./helpers/global-setup"); // file contents above

helpers.fixupIndex();

describe("Position of modules", () => {
.
.
.
}
helpers.restoreIndex();

the testcase works perfectly when the two files are modified in advance.

Testcase setup in jest, resolved