Creating an export function with JavaScript?

I'm trying to set up an export function in JavaScript for a packaged web app that turns a string stored in localStorage into a plain text file for downloading. As JavaScript does not have access to the computer's file-system, I'd like to set it up so that it create a blank text file (or, failing that, a simple HTML page) and open in in the web-browser; as it wouldn't be accessing any file-systems I was hoping this would be possible.

I was thinking of using a Data URI scheme to open the localStorage as plain text, such as the following:

function exportFile() {
 window.open("data:text/plain;charset=utf-8," + localStorage.WebAppData);
};

But 开发者_如何学编程it's much slower than I expected, which I guess is because it's sticking the whole document in the URL box. Though probably not an issue with the code, some web browsers, like Google Chrome, won't let me save the resulting file. (And for some reason all the line-breaks have turned into spaces....)

Any suggestions to fix these problems or better ways of doing a similar function will be greatly appreciated!

Did you try something like:

window.open("data:text/plain;charset=utf-8," + localStorage.WebAppData);

For the download, I guess you need a round trip to a server, that will set a mime/type that will make the download box to pop up.

EDIT:
If you use localStorage, may be window.postMessage is available in your environment and could help for speed.

In order to retain line-breaks in the data exported with window.open you may wrap up your data with encodeURI:

var data1 = "Line \n break. And \r\n another one";
window.open("data:application/octet-stream, " + encodeURI(data1));

Otherwise you may export your data encoded in base64 with the btoa function:

var data1 = "Line \n break. And \r\n another one";
window.open("data:application/octet-stream;base64, " + btoa(data1));

Not really a solution, rather a work-around, but your question and the answer by @Mic lead me down this route:

Just use data:text/html as then you can put in line breaks using <br />

1. I tried everything else (all combinations of unicode characters, etc, ) to get line breaks in text/plain but couldn't get them to show up. document.write() and document.body.textContent(), etc also suffer from the same problem. Line breaks just get ignored.
2. Since Chrome won't let you save the popup window anyway, the only way to get text out of it is copy and paste so there is no benefit of using text/plain over text/html
3. In web browsers that will let you save the page (Firefox) you can choose to save the page as text, rather than HTML and so you still get the same end result.

EDIT: This approach works in Chrome, but not Firefox

win = window.open("", "win")
win.document.body.innerText = "Line \n breaks?"

Have to use innerText though. InnerHTML or textContent remove the line breaks. This works on both:

win = window.open("", "win")
win.document.body.innerHTML = "<pre>Line \n breaks?</pre>"

So perhaps you could just wrap everything in <pre> tags? Although I guess both of these have the same "problem" as the `
suggestion in that it's actually creating a HTML document rather than a text/plain one.