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function queryString() {

	var query_string = {};
	var query = window.location.search.substring(1);
	var vars = query.split("&");
	for (var i=0;i<vars.length;i++) {
		var pair = vars[i].split("=");
				// If first entry with this name
		if (typeof query_string[pair[0]] === "undefined") {
			query_string[pair[0]] = decodeURIComponent(pair[1]);
				// If second entry with this name
		} else if (typeof query_string[pair[0]] === "string") {
			var arr = [ query_string[pair[0]],decodeURIComponent(pair[1]) ];
			query_string[pair[0]] = arr;
				// If third or later entry with this name
		} else {
			query_string[pair[0]].push(decodeURIComponent(pair[1]));
		}
	} 
		return query_string;
}


//URL parameter ?f=string

if(queryString().f == "string"){
	// do stuff
}