JSFiddle - React, Tailwind, and code Playground

JavaScript

var string = 'p1=v11&p1=v12&p2=v21&p2=v22&p2=v23&p3=v31';
    var rawParams = string.split('&');
    var params = [];
    var param = '';
    
    for (var i in rawParams) {
        param = rawParams[i].split('=');
        if ('p2' == param[0]) {
            params.push(param[1]);
        }
    }

console.log(params);