How do a I terminate a co.js loop after execution? -

whenever run co generator loop, nothing happens after execution process hangs. how terminate after completion?

co(function *() {     // code; }) 

co() returns promise. return loop , handle .then(). here return string "done", send console.log, , terminate process. errors printed console.

co(function *() {     return "done"; }).then(   res => { console.log(res); process.exit() } ).catch(err => console.error(err) );