How to set a file name using window.open

You can achieve this using the download attribute for <a> elements. For example:

<a href="1251354216241621.txt" download="your-foo.txt">Download Your Foo</a>

This attribute indicates that the file should be downloaded (instead of displayed, if applicable) and specifies which filename should be used for the downloaded file.

Instead of using window.open() you could generate an invisible link with the download attribute and .click() it.

var str = "Name, Price\nApple, 2\nOrange, 3";
var uri = 'data:text/csv;charset=utf-8,' + str;

var downloadLink = document.createElement("a");
downloadLink.href = uri;
downloadLink.download = "data.csv";

document.body.appendChild(downloadLink);
downloadLink.click();
document.body.removeChild(downloadLink);

Unfortunately this isn't supported in all browsers, but adding it won't make things worse for other browsers: they'll continue to download the files with useless filenames. (This assumes that you're using a MIME type is that their browser attempts to download. If you're trying to let the user download an .html file instead of displaying it, this won't do you any good in unsupported browsers.)

That does not work in latest Chrome, I have modified that and the following code will work fine,

 $("#download_1").on('click', function() {
    var json_pre = '[{"Id":1,"UserName":"Sam Smith"},{"Id":2,"UserName":"Fred Frankly"},{"Id":1,"UserName":"Zachary Zupers"}]';
    var json = $.parseJSON(json_pre);
   
    var csv = JSON2CSV(json);
    var downloadLink = document.createElement("a");
    var blob = new Blob(["\ufeff", csv]);
    var url = URL.createObjectURL(blob);
    downloadLink.href = url;
    downloadLink.download = "data.csv";

    document.body.appendChild(downloadLink);
    downloadLink.click();
    document.body.removeChild(downloadLink);
});

So when you click on download_1 id button then it will create an invisible download link and click that. I have used another function to prepare js.

The JSON2CSV function is as follows:

function JSON2CSV(objArray) {
    var array = typeof objArray != 'object' ? JSON.parse(objArray) : objArray;
    var str = '';
    var line = '';

    for (var i = 0; i < array.length; i++) {
        var line = '';

        for (var index in array[i]) {
           line += array[i][index] + ',';
        }

        line = line.slice(0, -1);
        str += line + '\r\n';
    }
    return str;
}

Hope it will help others :)

A shorter version of accepted one (for my case had to use unicode)

var link = document.createElement("a");
link.href = 'data:text/csv,' + encodeURIComponent("algún texto");
link.download = "Example.csv";
link.click();