const str  = 'mycookie=test';

const spl = str.split(';');

const x = spl.map(v => v.split('='))
.map(([k, ...vs]) => [k, vs.join('=')]).reduce((acc, [k, v]) ={
       acc[k.trim()] = decodeURIComponent(v);
       return acc;
 }, {});

console.log(x);