onrejected vs catch in Promise [duplicate]

Question

What is the difference between catch and then(_,onRejected) in ES6 Promise? I just know that onRejected doesn't handle the rejected state of inner Promise.

Promise.resolve().then(() => {
    return new Promise((resolve,reject) => {
      throw new Error('Error occurs');
    }); 
},er => console.log(er)); //Chrome throws `Uncaught (in promise)`

Promise.resolve().then(() => {
    return new Promise((resolve,reject) => {
      throw new Error('Error occurs');
    }); 
}).catch(er => console.log(er)); //Error occurs

Answer 1

Your first piece of code wont catch the error because the error handler is in the same .then where the error is thrown

---

As for your question

.catch(onRejected);

is identical to

.then(null, onRejected);

not sure what

.then(_, onRejected);

would do, depends on what _ is I guess