JSFiddle - React, Tailwind, and code Playground

JavaScript

function getURLParam(key,target){
        var values = [];
        if(!target){
            target = location.href;
        }

		key = key.replace(/[\[]/, "\\\[").replace(/[\]]/, "\\\]");
        
        var pattern = key + '=([^&#]+)';
        var o_reg = new RegExp(pattern,'ig');
		while(true){
			var matches = o_reg.exec(target);
			if(matches && matches[1]){
				values.push(matches[1]);
			}
			else{
				break;
			}
		}
        if(!values.length){
             return null;   
        }
        else{
            return values.length == 1 ? values[0] : values;
        }
    }
	
	var str = 'http://www.example.com?name[]=hello&name[]=world&var1=stam';

	console.log(getURLParam('name[]',str));
	console.log(getURLParam('var1',str));