Choose object attribute based on type of passed object

Issue

In the code below:

enum FirstSourceTopicAliases {
    Topic = 'topic',
    TopicText = 'topic_text',
    TopicLink = 'topic_link',
}
enum SecondSourceTopicAliases {
    Topic = 'title',
    TopicText = 'body',
    TopicLink = 'link',
}
interface FirstSourceTopic {
    [FirstSourceTopicAliases.Topic]: string,
    [FirstSourceTopicAliases.TopicText]: string,
    [FirstSourceTopicAliases.TopicLink]: string,
}
interface SecondSourceTopic {
    [SecondSourceTopicAliases.Topic]: string,
    [SecondSourceTopicAliases.TopicText]: string,
    [SecondSourceTopicAliases.TopicLink]: string,
}

type Topic = FirstSourceTopic | SecondSourceTopic

const parseTopic = (topic: Topic) => {
    const parsedTopic = {
        topic: 'parsed' + topic.topic // or topic.title
        topicText: 'parsed' + topic.topic_text // or topic.body
        topicLink: 'parsed' + topic.topic_link, // or topic.link
    }

    return parsedTopic
}

const firstSourceTopic: FirstSourceTopic = {
    topic: 'some topic',
    topic_text: 'some topic text',
    topic_link: 'some topic link',
}
const secondSourceTopic: SecondSourceTopic = {
    title: 'some topic',
    body: 'some topic text',
    link: 'some topic link',
}

const parsedTopic = parseTopic(firstSourceTopic)
console.log(parsedTopic)

Is it possible to make parseRow function generic enough so that it knows what key to extract from topic object that is passed to it? Currently the compiler will complain if I choose one of the attributes because the attribute is not present on the other type.

Solution

You could add a disciminator field property to disambiguate between your types (currently you have no intersection), e.g.

enum FirstSourceTopicAliases {
    Topic = 'topic',
    TopicText = 'topic_text',
    TopicLink = 'topic_link',
}
enum SecondSourceTopicAliases {
    Topic = 'title',
    TopicText = 'body',
    TopicLink = 'link',
}
interface FirstSourceTopic {
    kind:'first',
    [FirstSourceTopicAliases.Topic]: string,
    [FirstSourceTopicAliases.TopicText]: string,
    [FirstSourceTopicAliases.TopicLink]: string,
}
interface SecondSourceTopic {
       kind:'second',
    [SecondSourceTopicAliases.Topic]: string,
    [SecondSourceTopicAliases.TopicText]: string,
    [SecondSourceTopicAliases.TopicLink]: string,
}

type Topic = FirstSourceTopic | SecondSourceTopic

const parseTopic = <T extends FirstSourceTopic | SecondSourceTopic>(topic: T) => {
    if (topic.kind==='first') {
        return {
        topic: 'parsed' + topic.topic ,
        topicText: 'parsed' + topic.topic_text, 
        topicLink: 'parsed' + topic.topic_link, 
    }}
    return {
        topic: 'parsed' + topic.title ,
        topicText: 'parsed' + topic.body, 
        topicLink: 'parsed' + topic.link, 
    }
}

const firstSourceTopic: FirstSourceTopic = {
    kind:'first',
    topic: 'some topic',
    topic_text: 'some topic text',
    topic_link: 'some topic link',
}
const secondSourceTopic: SecondSourceTopic = {
    kind:'second',
    title: 'some topic',
    body: 'some topic text',
    link: 'some topic link',
}

const parsedTopic = parseTopic(firstSourceTopic)
console.log(parsedTopic)

Playground

Answered By - Damian Green