How to break promise chain on error

Question

Consider this snippet

fetch(`http://${api.host}:${api.port}/user`)
  .then(function(data) {
    return data.json();
  }, function(err) {
    throw new Error(`Cou         


        

Answer 1

No, throw does not jump to catch. It does reject the promise, and all error handlers installed on it will be invoked. In your case, that's the error handler installed by the then call. Notice that .catch(handler) is just sugar for .then(null, handler).

Your current code works like

try {
    try {
        try {
            var data = fetch(`http://${api.host}:${api.port}/user`)
        } catch(err) {
            throw new Error(`Couldn\'t fetch user data from server: ${err.message}`);
        }
        var eparkUser = data.json();
    } catch(err) {
        throw new Error(`Couldn't parse returned json: ${err.message}`);
    }
    for (var key in eparkUser) {
        if (eparkUser.hasOwnProperty(key) && !user.hasOwnProperty(key)) {
            user[key] = eparkUser[key];
        }
    }
    done(null, user);
} catch(e) {
    done(e);
}

To solve your problem, you'll need to nest your handlers, and install the JSON-parse-handler only on that particular promise:

fetch(`http://${api.host}:${api.port}/user`)
    .then(function (data) {
        return data.json()
            .then(function (eparkUser) {
                for (var key in eparkUser) {
                    if (eparkUser.hasOwnProperty(key) && !user.hasOwnProperty(key)) {
                        user[key] = eparkUser[key];
                    }
                }
                return user;
            }, function(err) {
                throw new Error(`Couldn't parse returned json: ${err.message}`);
            });
    }, function(err) {
        throw new Error(`Couldn\'t fetch user data from server: ${err.message}`);
    })
    .then(done.bind(null, null), done);