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Questions about fs.writeFile

I want to use fs.WriteFile in my JS project. I am building an algorithm that outputs random data and I want to give the user the opportunity to save the data as a txt file. I have been able to implement fs.WriteFile in my project, but I have a couple of questions on how to proceed next as the function remains somewhat unclear.
How do I specify that I want to include the contents of various vars? Is it as simple as data = let1 + let2 + let3 and all of the data will be included?
can I add the current date and time in the .txt file name? If so, how?
How do I tell writeFile to save the contents to a .txt file and open a download blob so that people can specify their own download locations?
Thanks in advance!
I've tried looking at basic documentation but its mainly the same: a template using a simple string that saves into the same directory, which is what I don't want.

For you first question, you are correct. You can just combine different string variables into a larger string variable. See the documentation for string concatenation for more information.
For your second question, yes you can. You can get the current date and time with new Date() and turn it into a variety of formats. For file names, using mydate.toISOString() will probably be the most clean.
Here's an example of both of these in practice:
import fs from 'fs';
// Here's some data that we want to put in the file.
const name = "Bob";
const age = 43;
// Create the data we want to put in our file.
const data = name + '\n' + age;
// Let's grab the date and use it as part of our file name.
const date = new Date();
const fileName = `${date.toISOString()}.txt`;
// Call fs.writeFile to put the data in the file.
fs.writeFile(fileName, data, () => {
console.log(`Wrote data to ${fileName}.`);
});
Your third question is more complicated and probably worth a separate post. fs.writeFile can't do this for you. You'll have to come up with some mechanism for the user to enter their own file name and build off of that.
Edit:
To address your question in the comments, you might be a little confused with how NodeJS works. NodeJS runs on the server and doesn't have any way to deal with buttons or UIs by default like browser JavaScript does. It might be helpful to look at the differences between the two. So you won't be able to save it to the downloads folder on a button click.
With that said, we can save the file to the user's Downloads folder with the same script I posted above by adding the path to the Downloads folder to the beginning of the file name.
Here's the code above adjusted to do that:
import fs from 'fs';
import os from 'os'; // NEW
import path from 'path'; // NEW
const name = "Bob";
const age = 43;
const data = name + '\n' + age;
const date = new Date();
const fileName = `${date.toISOString()}.txt`;
// Get the user's home directory.
const homedir = os.homedir();
// Append the Downloads directory and fileName to the user's home directory.
const fullPath = path.join(homedir, 'Downloads', fileName);
// Use fullPath here instead of fileName.
fs.writeFile(fullPath, data, () => {
console.log(`Wrote data to ${fileName}.`);
});

Convert plain text to .yml in javascript and return it

Im creating a file upload system which sends a string to my api made with express. I am trying to make the api take the string and create a yml file and return the created yml.
Currently I have
fs.writeFileSync('temp.yml', file);
const actualFile = fs.readFileSync('./temp.yml', 'utf8');
const data = fs.createReadStream('./temp.yml');
// delete file fs.unlinkSync('./temp.yml');
return data;
While this gets the job done, I dont believe its the best way. How can I return the read stream or the file without creating a file in the directory? Thanks
Create+Read+Stream+without+creating+file+javascript

How to open and write data into file from API call using node.js

I wrote an API and the response from that API is array of data. Whenever the response coming from that API i want to store that response in file that is.txt format. I tried but it shows an error like "No such directory or NO such path". How to create file and write data into file from API using node.js. This is the code i wrote :
exports.Entry = functions.https.onRequest((req, res) => {
var fs = require('fs');
var a = ['6', '7', '8'];
var b = ['22', '27', '20'];
var eachrecord = [];
for (var i = 0; i < a.length; i++) {
eachrecord += a + b;
}
console.log("eachrecord is", eachrecord);
//Writing each record value into file
fileWriteSync('./filewriting1.txt');
function fileWriteSync(filePath){
var fd = fs.openSync(filePath,'w');
var length = eachrecord.length;
for(i = 0;i<length;i++){
var eachrecordwrite = fs.writeSync(fd,eachrecord[i] + '\n',null,null);
console.log("hii",eachrecord[i]);
}
fs.closeSync(fd);
}
});
How to write data into file from API using node.js

You can only write files to os.tmpdir(), which is /tmp on Cloud Functions. /tmp is a memory based filesystem. Everything else is read-only. If you don't intend to do anything with that written file, it will consume memory indefinitely. You should always delete files written to /tmp before the function terminates. Writing a file to memory like this is almost certainly not the best solution to a problem, unless there is a consumer for that content that can only read it off the local filesystem.
Since you haven't really said what problem you're trying to solve, it's not possible to say what you could be doing instead (that's something for a different question). But anyway, you can only write to /tmp.

Javascript, Typescript, Angular 5 - Open and read file

I am working with Angular 5, I have an application in which I need to read an AMP HTML file as text. This file is contained in a component and should only be accessed from this component.
I would like to be able to open the file in read-only by giving its name.
I'm actually searching for something like this:
let file = open('amp.html');
Is it possible? If not how can I do to achieve this?

If you're writing browserside JS
You can't just simply read a file. The JS is running on your browser, and you need to think about where you're getting that file from.
If the file is on a server, you need to fetch that file from the server first by making a request for it.
If you're reading a file on the user's computer, you're gonna be using the File API on the browser to allow the user to select that file.
If you're writing backend JS
Assuming you're using NodeJS, you can conduct file operations like you would with other programming languages. Check out the fs module

If i understand you correct, you can read it as text like this:
function readFile(file){
var raw = new XMLHttpRequest(); // create a request
raw.open("GET", file, false); // open file
raw.onreadystatechange = function (){ // file is ready to read
if(raw.readyState === 4){
if(raw.status === 200 || raw.status == 0){
var allText = raw.responseText;
alert(allText); // can be also console.logged, of course.
}
}
}
raw.send(null); // return control
}
usage:
readFile('link.html')
I solved this issue thankfully to this question.

Reading/Writing to a file in javascript

I want to read and write to a file in a specific way.
An example file could be:
name1:100
name2:400
name3:7865786
...etc etc
What would be the best way to read this data in and store in, and eventually write it out?
I don't know which type of data structure to use? I'm still fairly new to javascript.
I want to be able to determine if any key,values are matching.
For example, if I were to add to the file, I could see that name1 is already in the file, and I just edit the value instead of adding a duplicate.

You can use localStorage as a temporary storage between reads and writes.
Though, you cannot actually read and write to a user's filesystem at will using client side JavaScript. You can however request the user to select a file to read the same way you can request the user to save the data you push, as a file.
localStorage allow you to store the data as key-value pairs and it's easy to check if an item exists or not. Optionally simply use a literal object which basically can do the same but only exists in memory. localStorage can be saved between sessions and navigation between pages.
// set some data
localStorage.setItem("key", "value");
// get some data
var data = localStorage.getItem("key");
// check if key exists, set if not (though, you can simply override the key as well)
if (!localStorage.getItem("key")) localStorage.setItem("key", "value");
The method getItem will always return null if the key doesn't exist.
But note that localStorage can only store strings. For binary data and/or large sizes, look into Indexed DB instead.
To read a file you have to request the user to select one (or several):
HTML:
<label>Select a file: <input type=file id=selFile></label>
JavaScript
document.getElementById("selFile").onchange = function() {
var fileReader = new FileReader();
fileReader.onload = function() {
var txt = this.result;
// now we have the selected file as text.
};
fileReader.readAsText(this.files[0]);
};
To save a file you can use File objects this way:
var file = new File([txt], "myFilename.txt", {type: "application/octet-stream"});
var blobUrl = (URL || webkitURL).createObjectURL(file);
window.location = blobUrl;
The reason for using octet-stream is to "force" the browser to show a save as dialog instead of it trying to show the file in the tab, which would happen if we used text/plain as type.
So, how do we get the data between these stages. Assuming you're using key/value approach and text only you can use JSON objects.
var file = JSON.stringify(localStorage);
Then send to user as File blob shown above.
To read you will have to either manually parse the file format if the data exists in a particular format, or if the data is the same as you save out you can read in the file as shown above, then convert it from string to an object:
var data = JSON.parse(txt); // continue in the function block above
Object.assign(localStorage, data); // merge data from object with localStorage
Note that you may have to delete items from the storage first. There is also the chance other data have been stored there so these are cases that needs to be considered, but this is the basis of one approach.
Example
// due to security reasons, localStorage can't be used in stacksnippet,
// so we'll use an object instead
var test = {"myKey": "Hello there!"}; // localStorage.setItem("myKey", "Hello there!");
document.getElementById("selFile").onchange = function() {
var fileReader = new FileReader();
fileReader.onload = function() {
var o = JSON.parse(this.result);
//Object.assign(localStorage, o); // use this with localStorage
alert("done, myKey=" + o["myKey"]); // o[] -> localStorage.getItem("myKey")
};
fileReader.readAsText(this.files[0]);
};
document.querySelector("button").onclick = function() {
var json = JSON.stringify(test); // test -> localStorage
var file = new File([json], "myFilename.txt", {type: "application/octet-stream"});
var blobUrl = (URL || webkitURL).createObjectURL(file);
window.location = blobUrl;
}
Save first: <button>Save file</button> (<code>"myKey" = "Hello there!"</code>)<br><br>
Then read the saved file back in:<br>
<label>Select a file: <input type=file id=selFile></label>

Are you using Nodejs? Or browser javascript?
In either case the structure you should use is js' standard object. Then you can turn it into JSON like this:
var dataJSON = JSON.stringify(yourDataObj)
With Nodejs, you'll want to require the fs module and use one of the writeFile or appendFile functions -- here's sample code:
const fs = require('fs');
fs.writeFileSync('my/file/path', dataJSON);
With browser js, this stackoverflow may help you: Javascript: Create and save file

I know you want to write to a file, but but consider a database instead so that you don't have to reinvent the wheel. INSERT ... ON DUPLICATE KEY UPDATE seems like the logical choice for what you're looking to do.

For security reasons it's not possible to use JavaScript to write to a regular text or similar file on a client's system.
However Asynchronous JavaScript and XML (AJAX) can be used to send an XMLHttpRequest to a file on the server, written in a server-side language like PHP or ASP.
The server-side file can then write to other files, or a database on the server.
Cookies are useful if you just need to save relatively small amounts of data locally on a client's system.
For more information have a look at
Read/write to file using jQuery