Handling Js promise rejection

Question

How do you handle an error (eg. "new error" below) that is outside of the promise?

function testError() {
    throw new Error("new error") // how to handle this?
    var p123 = new Promise(function(resolve, reject) {
         resolve(123)
    });
    return p123
};

testError().catch(err => {
        return err;  // code doesn't come here
    })
    .then(ok => {
        console.log(ok)
    });

Answer 1

If you're not sure whether a function will throw (or return a value) synchronously, you can call it using .then(). This will wrap it so that the result will be handled consistently no matter how it is produced:

function testError() {
  throw new Error("new error") // how to handle this?
  var p123 = new Promise(function(resolve, reject) {
    resolve(123)
  });
  return p123
};

Promise.resolve()
  .then(testError)
  .catch(err => {
    console.error(err);
    return err; 
  })
  .then(ok => {
    console.log(ok.message)
  });