Imagine you have an HTML table, for example something like this:

<table>
    <tr><th>Name</th><th>Age</th><th>Country</th></tr>
    <tr><td>Geronimo</td><td>26</td><td>France</td></tr>
    <tr><td>Natalia</td><td>19</td><td>Spain</td></tr>
    <tr><td>Silvia</td><td>32</td><td>Russia</td></tr>
</table>

And you want to download it as a CSV table format.

First of all, you need to transform from HTML to CSV.

	var csv = [];
	var rows = document.querySelectorAll("table tr");
	
    for (var i = 0; i < rows.length; i++) {
		var row = [], cols = rows[i].querySelectorAll("td, th");
		
        for (var j = 0; j < cols.length; j++) 
            row.push(cols[j].innerText);
        
		csv.push(row.join(","));		
	}

    // Download CSV
    download_csv(csv.join("\n"), filename);

After that, you can download it using Blob and a link.

    var csvFile;
    var downloadLink;

    // CSV FILE
    csvFile = new Blob([csv], {type: "text/csv"});

    // Download link
    downloadLink = document.createElement("a");

    // File name
    downloadLink.download = filename;

    // We have to create a link to the file
    downloadLink.href = window.URL.createObjectURL(csvFile);

    // Make sure that the link is not displayed
    downloadLink.style.display = "none";

    // Add the link to your DOM
    document.body.appendChild(downloadLink);

    // Launch
    downloadLink.click();

Here a real example: