04.iii Promise.finally()

Finally method - a callback to be invoked when a promise is settled (either fulfilled, or rejected):

promise
    .then(result => {···})
    .catch(error => {···})
    .finally(() => {···});

Features:

• A finally’s callback is always executed

• A finally callback will not receive any argument

• Preserve Promise state

  • Promise.resolve(2).finally(() => {}) will be resolved with 2.

  • Promise.reject(3).finally(() => {}) will be rejected with 3.

• A throw (or returning a rejected promise) in the finally callback will reject the new promise with that rejection reason.

promise.finally(() => {
    «statements»
});

// Is equal to:
promise.then(
    result => {
        «statements»
        return result;
    },
    error => {
        «statements»
        throw error;
    }
);

The essential use case here is cleanup - I want to hide the "loading" spinner on my AJAX request, or I want to close any file handles I’ve opened, or I want to log that an operation has completed regardless of whether it succeeded or not.