Remove plus sign (+) in URL query string

You can use replace() for this purpose

var dateString = 'Friday+September+13th';
var s = dateString .replace(/\+/g, ' ');

Parsing strings using regex is often prone to so many errors. Thankfully all modern browsers provide URLSearchParams to handle params from url strings in a proper way:

var params = new URLSearchParams(window.location.search);
var value = params.get('ks4day');
// "Friday September 13th"

Ps: There is also a good polyfill for old browsers.

Although Bibhu's answer will work for this one case, you'll need to add decodeURIComponent if you have encoded characters in your URI string. You also want to make sure you do the replace before the decode in case you have a legitimate + in your URI string (as %2B).

I believe this is the best general way to do it:

var x = qs("ks4day");        // 'Friday+September+13th'
x = x.replace(/\+/g, '%20'); // 'Friday%20September%2013th'
x = decodeURIComponent(x);   // 'Friday September 13th'

Here's an example of when it might be useful:

var x = '1+%2B+1+%3D+2'; 
x = x.replace(/\+/g, '%20'); // '1%20%2B%201%20%3D%202'
x = decodeURIComponent(x);   // '1 + 1 = 2'

Have you tried https://www.npmjs.com/package/querystring ?

import { parse } from 'querystring'; 
parse('ks4day=Friday+September+13th')

returns

{ 'ks4day': 'Friday September 13th' }

Assuming you are using something like Webpack that knows how to process import statements