How to delete a cookie?

Try this:

function delete_cookie( name, path, domain ) {
  if( get_cookie( name ) ) {
    document.cookie = name + "=" +
      ((path) ? ";path="+path:"")+
      ((domain)?";domain="+domain:"") +
      ";expires=Thu, 01 Jan 1970 00:00:01 GMT";
  }
}

You can define get_cookie() like this:

function get_cookie(name){
    return document.cookie.split(';').some(c => {
        return c.trim().startsWith(name + '=');
    });
}

Here a good link on Quirksmode.

function setCookie(name,value,days) {
    var expires = "";
    if (days) {
        var date = new Date();
        date.setTime(date.getTime() + (days*24*60*60*1000));
        expires = "; expires=" + date.toUTCString();
    }
    document.cookie = name + "=" + (value || "")  + expires + "; path=/";
}
function getCookie(name) {
    var nameEQ = name + "=";
    var ca = document.cookie.split(';');
    for(var i=0;i < ca.length;i++) {
        var c = ca[i];
        while (c.charAt(0)==' ') c = c.substring(1,c.length);
        if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
    }
    return null;
}
function eraseCookie(name) {   
    document.cookie = name+'=; Max-Age=-99999999;';  
}

Some of the other solutions might not work if you created the cookie manually.

Here's a quick way to delete a cookie:

document.cookie = 'COOKIE_NAME=; Max-Age=0; path=/; domain=' + location.host;

If this doesn't work, try replacing location.host with location.hostname in the snippet above.

I had trouble deleting a cookie made via JavaScript and after I added the host it worked (scroll the code below to the right to see the location.host). After clearing the cookies on a domain try the following to see the results:

if (document.cookie.length==0)
{
 document.cookie = 'name=example; expires='+new Date((new Date()).valueOf()+1000*60*60*24*15)+'; path=/; domain='+location.host;

 if (document.cookie.length==0) {alert('Cookies disabled');}
 else
 {
  document.cookie = 'name=example; expires=Thu, 01 Jan 1970 00:00:00 GMT; path=/; domain='+location.host;

  if (document.cookie.length==0) {alert('Created AND deleted cookie successfully.');}
  else {alert('document.cookies.length = '+document.cookies.length);}
 }
}

For people who just want 1 line of code to delete a cookie:

If you created a cookie, for example in a web browser console with document.cookie = "test=hello"

You can delete it with:

document.cookie = "test=;expires=" + new Date(0).toUTCString()

Or if you prefer to write the UTC date directly:

document.cookie = "test=;expires=Thu, 01 Jan 1970 00:00:00 GMT"

If you are on a different path than the cookie (for example if you want to delete a cookie that is used on all paths), you can add path=/; after test=; and if you are on a different domain (for example when a cookie is set for all subdomains by using .example.com instead of www.example.com), you can add domain=.example.com; after test=;.

Update: instead of expires=..., using Max-Age=0 like in other answers works also (tested with Firefox).

I use this on my websites that works on Chrome and Firefox.

function delete_cookie(name) { document.cookie = name +'=; Path=/;  Domain=' + location.host +  '; Expires=Thu, 01 Jan 1970 00:00:01 GMT; SameSite=None; Secure' }

if ("JSESSIONID".equals(cookie.getName()) || "LtpaToken2".equals(cookie.getName())) {
                cookie.setValue("");
                cookie.setPath("/");
                cookie.setMaxAge(0);
                cookie.setHttpOnly(true);
                response.addCookie(cookie);
}