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Javascript file input

I am trying to make a JS program using a text file on the client side. The thing is that it was difficult to find out how to do such a basic functionality because it is not a desired feature. I want to open a text file which is not in the local directory of a user but in the directory with the html file. So, what I want is the JS version of this code (which is in python)
# Open a file
fo = open("foo.txt", "wb")
fo.write( "Python is a great language.\nYeah its great!!\n");
# Close opend file
fo.close()
Thanks!

You can read a file with FileReader() (as a result of a user selecting files) but fortunately we can't modify client's or server's files with JavaScript in client side (there is no FileWriter()). To do that, you should upload the file to the server (through Ajax or a simple html form) and modify it with server side code.

Use Html5 to read Excel file

I need to read excel file data when upload the file .
Is there any way that can use html5 to read excel file data when file uploading in client side??
heard about
var reader = new FileReader();
any way that we can use this

Referring to #mituw16 comment on the question, take a look at the following question:
How to parse Excel file in Javascript/HTML5
As a seperate response to work from, I would suggest using a plugin like:
https://github.com/SheetJS/js-xlsx
To iterate through the spreadsheet and save the information in your database.

How to load the contents of a local text file by name using Javascript and HTML 5?

Working in Chrome, loading a local html or JS file.
I found many examples of how to load a file that is selected using the Choose File input.
However, didn't figure out how to do it given a file name without using the Choose File input.
The Choose File input returns a File object.
How to create the File object without the Choose File input?
From the File API:
new File(
Array parts,
String filename,
BlobPropertyBag properties
);
But didn't figure out what the parts and properties would be.
Edit: Use case:
I have code coverage results generated as part of a test suite. It is stored as JSON (which is easy to read), but I need to display it with the source code.
So the feature is to load the source code and JSON data, and render them together on a web page using HTML and Javascript.
The file would be opened from the browser and lives on the local machine. There is no server.

The browser cannot load arbitrary files by name from your filesystem without special extensions or other shenanigans. This is a security policy to prevent random web sites from reading files from your hard disk as you browse the internet.
If you're down to do something special like if you want to write a chrome app, you could get access to some nice APIs for accessing the filesystem:
https://developer.chrome.com/apps/fileSystem

The File constructor doesn't read a file from the harddrive, but rater make a virtual file, consider this:
var file = new File(["some", "content"], "/tmp/my-name.txt");
var reader = new FileReader();
reader.onload = function() {
console.log(reader.result); // somecontent
};
No file will be read or stored on the clients machine.
If you are talking about creating files in nodejs then you should take a look at fs.

For security reasons all browsers don't support predefined values on file fields so the answer is you can't.

Upload file inside chrome extension

I need to add a browse button inside my Chrome extension. Users click it and choose a file. I then want to retrieve the file contents (bytes) and do some work on it.
I don't want to have to submit the file to a remote server and then get the response (if that's even doable from inside a Chrome extension), it should be all client-side.
Is this doable inside Chrome extensions?

You should be looking at the FileReader API.
The FileReader object lets web applications asynchronously read the contents of files (or raw data buffers) stored on the user's computer, using File or Blob objects to specify the file or data to read.
A very good basic example of using this interface is in this question.
A minimal example: suppose that you have an <input type="file" id="file"> with a text file selected.
var file = document.getElementById("file").files[0];
var reader = new FileReader();
reader.onload = function(e){
console.log(e.target.result);
}
reader.readAsText(file);
If you need methods other than reading as text (i.e. binary data), see the docs.
Also, this is a good overview: Using files from web applications

Regarding your question it is totally feasible to load and process a file within an extension. I implemented it using message passing https://developer.chrome.com/docs/extensions/mv3/messaging/.
Here is an example of how you can implement it, in my case I used the input file to load an excel. This is my public repo.
https://github.com/juanmachuca95/gomeetplus

how to write the user inputted values into the csv file or excel file in html page

How can i export the values of the text box in html to a csv or a excel
.I have created an HTML Form which will be used from a local computer and want the form data to be saved in CSV file.
Each time the form is submitted, it should add a line in the CSV file.
This needs to be run locally so cannot use PHP or JSP.

You can use file handling technique through java script to write .csv file
var fso = new ActiveXObject("Scripting.FileSystemObject");
// 2=overwrite, true=create if not exist, 0 = ASCII
varFileObject = fso.OpenTextFile("C:\\myfile.csv", 2, true,0);
varFileObject.write("File handling in Javascript");
varFileObject.close();
for more information about how to use file handling in Javascript visit code project site