dewedev/node_modules/fs-monkey/docs/api/patchFs.md

patchFs(vol[, fs])

Rewrites Node's filesystem module fs with fs-like object.

• vol - fs-like object
• fs (optional) - a filesystem to patch, defaults to require('fs')

import {patchFs} from 'fs-monkey';

const myfs = {
    readFileSync: () => 'hello world',
};

patchFs(myfs);
console.log(require('fs').readFileSync('/foo/bar')); // hello world

You don't need to create fs-like objects yourself, use memfs to create a virtual filesystem for you:

import {vol} from 'memfs';
import {patchFs} from 'fs-monkey';

vol.fromJSON({'/dir/foo': 'bar'});
patchFs(vol);
console.log(require('fs').readdirSync('/')); // [ 'dir' ]

Promises API is supported as well:

import {vol} from 'memfs';
import {patchFs} from 'fs-monkey';

vol.fromJSON({'/dir/foo': 'bar'});
patchFs(vol);
require('fs').promises.readFile('/dir/foo', 'UTF-8').then(console.log); // bar