Joining more than two items returns invalid output

[nzx9]

let mut params = ["?vs_currency", vs_currency, "&days", days].join("=");  

// test
["?vs_currency", 'usd', "&days", 1].join("=");
// output: ?vs_currency=usd=&days=1
// this should be  ?vs_currency=usd&days=1

Joining more than two items returns invalid output