'then' and 'catch' called one after annother

Question

Why 'then' and 'catch' is both called if I add callback function?

function sendMail(data, callback) {

    sgMail
        .send(data)
        .then(() => {
            console.log('Email sent');
            callback(true);
        })
        .catch((error) => {
            console.error(error);
            callback(false);
        })
}

I'm sorry if its duplicate I just cant find an answer.

Answer 1

catch() will run if code in a previous then() throws an error.

Take this example:

const promise = new Promise((resolve) => {
  resolve({ data: true })
})

promise.then(() => {
    console.log('Email sent');
    throw new Error("throw an error")
})
.catch((error) => {
    console.error(error);
    console.log('Error was thrown in then() clause');
})

The promise resolve successfully, then() executes until it throws an error, then catch is run because an error was thrown.

---

However this example never runs the catch() because no error is ever thrown.

    const promise = new Promise((resolve) => {
      resolve({ data: true })
    })

    promise.then(() => {
        console.log('Email sent');
    })
    .catch((error) => {
        console.error(error);
        console.log('Error was thrown in then() clause');
    })