nancy[apply].js

const apply = (value, state) => {
    // Ignore subsequent calls to resolve and reject
    if (this.state === states.pending) {
        this.value = value;
        changeState(state);
    }
};

const getCallback = state => value => {
    // Unpack on resolve
    if (value instanceof Nancy && state === states.resolved) {
        value.then(value => apply(value, states.resolved));
        value.catch(value => apply(value, states.rejected));
        // Either 'then' or 'catch' will happen here, not both
        // No need for more ifs!
    } else {
        apply(value, state);
    }
};