Javascript append to file

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Javascript

Good day, Please i am new to javascript and i want to save the results calculated on an html in a text file using javascript.
i don't really know how to implement it.
The webpage has java code calculate some numbers and before displaying it in page it has 'return' in its code so i put my lame code before it and some times it displays well while sometimes the page just shows blank.
Please what i want to do is for the website to save all results in a text file. that is it appends to the file.
My code is not correct i know because i am just new to it. so should i place it before the return command in the code?

What I have tried:

const dir = dirname('C:/Users/user/Desktop/WOW');

  if (!existsSync(dir)) {
    mkdirSync(dir, { recursive: true });
    core.debug(`Created directory: ${dir}`);
  }
var jj = sa + p(trimmed) + ea + " ".repeat(addr.length-trimmed.length);
const fs = require('fs')
const filePath = 'file.txt'
 fs.appendFile(filePath, jj, (err) => {
  })