Is it possible to write data to file using only JavaScript?

You can create files in browser using Blob and URL.createObjectURL. All recent browsers support this.

You can not directly save the file you create, since that would cause massive security problems, but you can provide it as a download link for the user. You can suggest a file name via the download attribute of the link, in browsers that support the download attribute. As with any other download, the user downloading the file will have the final say on the file name though.

var textFile = null,
  makeTextFile = function (text) {
    var data = new Blob([text], {type: 'text/plain'});

    // If we are replacing a previously generated file we need to
    // manually revoke the object URL to avoid memory leaks.
    if (textFile !== null) {
      window.URL.revokeObjectURL(textFile);
    }

    textFile = window.URL.createObjectURL(data);

    // returns a URL you can use as a href
    return textFile;
  };

Here's an example that uses this technique to save arbitrary text from a textarea.

If you want to immediately initiate the download instead of requiring the user to click on a link, you can use mouse events to simulate a mouse click on the link as Lifecube's answer did. I've created an updated example that uses this technique.

  var create = document.getElementById('create'),
    textbox = document.getElementById('textbox');

  create.addEventListener('click', function () {
    var link = document.createElement('a');
    link.setAttribute('download', 'info.txt');
    link.href = makeTextFile(textbox.value);
    document.body.appendChild(link);

    // wait for the link to be added to the document
    window.requestAnimationFrame(function () {
      var event = new MouseEvent('click');
      link.dispatchEvent(event);
      document.body.removeChild(link);
    });

  }, false);

If you are talking about browser javascript, you can not write data directly to local file for security reason. HTML 5 new API can only allow you to read files.

But if you want to write data, and enable user to download as a file to local. the following code works:

    function download(strData, strFileName, strMimeType) {
    var D = document,
        A = arguments,
        a = D.createElement("a"),
        d = A[0],
        n = A[1],
        t = A[2] || "text/plain";

    //build download link:
    a.href = "data:" + strMimeType + "charset=utf-8," + escape(strData);


    if (window.MSBlobBuilder) { // IE10
        var bb = new MSBlobBuilder();
        bb.append(strData);
        return navigator.msSaveBlob(bb, strFileName);
    } /* end if(window.MSBlobBuilder) */



    if ('download' in a) { //FF20, CH19
        a.setAttribute("download", n);
        a.innerHTML = "downloading...";
        D.body.appendChild(a);
        setTimeout(function() {
            var e = D.createEvent("MouseEvents");
            e.initMouseEvent("click", true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
            a.dispatchEvent(e);
            D.body.removeChild(a);
        }, 66);
        return true;
    }; /* end if('download' in a) */



    //do iframe dataURL download: (older W3)
    var f = D.createElement("iframe");
    D.body.appendChild(f);
    f.src = "data:" + (A[2] ? A[2] : "application/octet-stream") + (window.btoa ? ";base64" : "") + "," + (window.btoa ? window.btoa : escape)(strData);
    setTimeout(function() {
        D.body.removeChild(f);
    }, 333);
    return true;
}

to use it:

download('the content of the file', 'filename.txt', 'text/plain');

Try

let a = document.createElement('a');
a.href = "data:application/octet-stream,"+encodeURIComponent("My DATA");
a.download = 'abc.txt';
a.click();

If you want to download binary data look here

Update

2020.06.14 I upgrade Chrome to 83.0 and above SO snippet stop works (reason: sandbox security restrictions) - but JSFiddle version works - here

Above answer is useful but, I found code which helps you to download text file directly on button click. In this code you can also change filename as you wish. It's pure javascript function with HTML5. Works for me!

function saveTextAsFile()
{
    var textToWrite = document.getElementById("inputTextToSave").value;
    var textFileAsBlob = new Blob([textToWrite], {type:'text/plain'});
    var fileNameToSaveAs = document.getElementById("inputFileNameToSaveAs").value;
      var downloadLink = document.createElement("a");
    downloadLink.download = fileNameToSaveAs;
    downloadLink.innerHTML = "Download File";
    if (window.webkitURL != null)
    {
        // Chrome allows the link to be clicked
        // without actually adding it to the DOM.
        downloadLink.href = window.webkitURL.createObjectURL(textFileAsBlob);
    }
    else
    {
        // Firefox requires the link to be added to the DOM
        // before it can be clicked.
        downloadLink.href = window.URL.createObjectURL(textFileAsBlob);
        downloadLink.onclick = destroyClickedElement;
        downloadLink.style.display = "none";
        document.body.appendChild(downloadLink);
    }

    downloadLink.click();
}

const data = {name: 'Ronn', age: 27};              //sample json
const a = document.createElement('a');
const blob = new Blob([JSON.stringify(data)]);
a.href = URL.createObjectURL(blob);
a.download = 'sample-profile';                     //filename to download
a.click();

Check Blob documentation here - Blob MDN to provide extra parameters for file type. By default it will make .txt file

In the case it is not possibile to use the new Blob solution, that is for sure the best solution in modern browser, it is still possible to use this simpler approach, that has a limit in the file size by the way:

function download() {
                var fileContents=JSON.stringify(jsonObject, null, 2);
                var fileName= "data.json";

                var pp = document.createElement('a');
                pp.setAttribute('href', 'data:text/plain;charset=utf-8,' + encodeURIComponent(fileContents));
                pp.setAttribute('download', fileName);
                pp.click();
            }
            setTimeout(function() {download()}, 500);

$('#download').on("click", function() {
  function download() {
    var jsonObject = {
      "name": "John",
      "age": 31,
      "city": "New York"
    };
    var fileContents = JSON.stringify(jsonObject, null, 2);
    var fileName = "data.json";

    var pp = document.createElement('a');
    pp.setAttribute('href', 'data:text/plain;charset=utf-8,' + encodeURIComponent(fileContents));
    pp.setAttribute('download', fileName);
    pp.click();
  }
  setTimeout(function() {
    download()
  }, 500);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<button id="download">Download me</button>

This is the top result in Google for "js how to write to local file" so it warrants a newer answer in light of new APIs being added in 2023 (and is available in shipping browsers like Chrome).

In a 'secure context', a webpage can now ask for access to a directory or local file. When you ask for access to a directory, the user is prompted by the browser to accept or reject the request.

Once they have accepted your request, you have access to the directory and all sub-directories. At this point, you can ask for write access. (It is up for the browser to interpret that request.)

// open a directory picker and wait for the user to pick a file
const directory = await showDirectoryPicker();

// list the files in the directory
for await (const fileEntity of directory.values()) {
    if (fileEntity.kind === "file") {
        console.log(fileEntity.name);
    }
}

For a given FileSystemFileHandle you can turn it into a FileSystemWritableFileStream by calling createWritable. In Chrome, the first call to createWritable prompts the user to grant the code access to write to the local file..

// write a file (and implicitly ask for write permission)
const writable = await fileEntity.createWritable();
await writable.write("Hello!");
await writable.close();

All modern browsers seem to support this API apart from Safari, which does not support createWritable.

Use the code by the user @useless-code above (https://stackoverflow.com/a/21016088/327386) to generate the file. If you want to download the file automatically, pass the textFile that was just generated to this function:

var downloadFile = function downloadURL(url) {
    var hiddenIFrameID = 'hiddenDownloader',
    iframe = document.getElementById(hiddenIFrameID);
    if (iframe === null) {
        iframe = document.createElement('iframe');
        iframe.id = hiddenIFrameID;
        iframe.style.display = 'none';
        document.body.appendChild(iframe);
    }
    iframe.src = url;
}

I found good answers here, but also found a simpler way.

The button to create the blob and the download link can be combined in one link, as the link element can have an onclick attribute. (The reverse seems not possible, adding a href to a button does not work.)

You can style the link as a button using bootstrap, which is still pure javascript, except for styling.

Combining the button and the download link also reduces code, as fewer of those ugly getElementById calls are needed.

This example needs only one button click to create the text-blob and download it:

<a id="a_btn_writetofile" download="info.txt" href="#" class="btn btn-primary" 
   onclick="exportFile('This is some dummy data.\nAnd some more dummy data.\n', 'a_btn_writetofile')"
>
   Write To File
</a>

<script>
    // URL pointing to the Blob with the file contents
    var objUrl = null;
    // create the blob with file content, and attach the URL to the downloadlink; 
    // NB: link must have the download attribute
    // this method can go to your library
    function exportFile(fileContent, downloadLinkId) {
        // revoke the old object URL to avoid memory leaks.
        if (objUrl !== null) {
            window.URL.revokeObjectURL(objUrl);
        }
        // create the object that contains the file data and that can be referred to with a URL
        var data = new Blob([fileContent], { type: 'text/plain' });
        objUrl = window.URL.createObjectURL(data);
        // attach the object to the download link (styled as button)
        var downloadLinkButton = document.getElementById(downloadLinkId);
        downloadLinkButton.href = objUrl;
    };
</script>

Here is a single-page local-file version for use when you need the extra processing functionality of a scripting language.

1. Save the code below to a text file
2. Change the file extension from '.txt' to '.html'
3. Right-click > Open With... > notepad
4. Program word processing as needed, then save
5. Double-click html file to open in default browser
6. Result will be previewed in the black box, click download to get the resulting text file

Code:

<!DOCTYPE HTML>
<HTML>
<HEAD>
</HEAD>
<BODY>
<SCRIPT>
    // do text manipulation here
    let string1 = 'test\r\n';
    let string2 = 'export.';
    
    // assemble final string
    const finalText = string1 + string2;
    
    // convert to blob
    const data = new Blob([finalText], {type: 'text/plain'});
    
    // create file link
    const link = document.createElement('a');
    link.innerHTML = 'download';
    link.setAttribute('download', 'data.txt');
    link.href = window.URL.createObjectURL(data);
    document.body.appendChild(link);
    
    // preview the output in a paragraph
    const htmlBreak = string => {
        return string.replace(/(?:\r\n|\r|\n)/g, '<br>');
    }
    const preview = document.createElement('p');
    preview.innerHTML = htmlBreak(finalText);
    preview.style.border = "1px solid black";
    document.body.appendChild(preview);
</SCRIPT>
</BODY>
</HTML>