[javascript] How to make a browser display a "save as dialog" so the user can save the content of a string to a file on his system?

How can I make a browser display a "save as dialog" so the user can save the content of a string to a file on his system?

For example:

var myString = "my string with some stuff";
save_to_filesystem(myString,"myString.txt");

Resulting in something like this:

Solution using only javascript

function saveFile(fileName,urlFile){
    let a = document.createElement("a");
    a.style = "display: none";
    document.body.appendChild(a);
    a.href = urlFile;
    a.download = fileName;
    a.click();
    window.URL.revokeObjectURL(url);
    a.remove();
}

let textData = `El contenido del archivo
que sera descargado`;
let blobData = new Blob([textData], {type: "text/plain"});
let url = window.URL.createObjectURL(blobData);
//let url = "pathExample/localFile.png"; // LocalFileDownload
saveFile('archivo.txt',url);

There is a new spec called the Native File System API that allows you to do this properly like this:

const result = await window.chooseFileSystemEntries({ type: "save-file" });

There is a demo here, but I believe it is using an origin trial so it may not work in your own website unless you sign up or enable a config flag, and it obviously only works in Chrome. If you're making an Electron app this might be an option though.

There is a javascript library for this, see FileSaver.js on Github

However the saveAs() function won't send pure string to the browser, you need to convert it to blob:

function data2blob(data, isBase64) {
  var chars = "";

  if (isBase64)
    chars = atob(data);
  else
    chars = data;

  var bytes = new Array(chars.length);
  for (var i = 0; i < chars.length; i++) {
    bytes[i] = chars.charCodeAt(i);
  }

  var blob = new Blob([new Uint8Array(bytes)]);
  return blob;
}

and then call saveAs on the blob, as like:

var myString = "my string with some stuff";
saveAs( data2blob(myString), "myString.txt" );

Of course remember to include the above-mentioned javascript library on your webpage using <script src=FileSaver.js>

Using execComand:

<input type="button" name="save" value="Save" onclick="javascript:document.execCommand('SaveAs','true','your_file.txt')">

In the next link: execCommand

This is possible using this cross browser javascript implementation of the HTML5 saveAs function: https://github.com/koffsyrup/FileSaver.js

If all you want to do is save text then the above script works in all browsers(including all versions of IE), using nothing but JS.