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Intersecting Incompatible Types

What happens when we intersect types with overlapping properties?

type Saiyan = {
  name: string;
  powerLevel: number;
};

type Human = {
  name: string;
  age: number;
};

type SaiyanHuman = Saiyan & Human;

We get this SaiyanHuman type that's the equivalent of:

type SaiyanHuman = {
  name: string;
  powerLevel: number;
  age: number;
};

It merges the properties of both Saiyan and Human, and because name overlaps, it safely combines the two types and appears once in the resulting type.

When Things Go Wrong

What happens if the name field were incompatible types? For example:

type Saiyan = {
  name: "goku" | "vegeta";
  powerLevel: number;
};

type Human = {
  name: "krillin" | "yamcha";
  age: number;
};

type SaiyanHuman = Saiyan & Human;

Now the name property can't possibly satisfy both! Humans must be krillin or yamcha, and Saiyans must be goku or vegeta. So, the name property in SaiyanHuman becomes never, which in turn makes the entire SaiyanHuman type never.

// Type '{}' is not assignable to type 'never'
const theLaneagen: SaiyanHuman = {};

It's TypeScript saying, "Hey, the SaiyanHuman type is impossible, do something else." Most of the time, the solution here is to redesign your types to avoid incompatible intersections and make sense.

Assignment

Support.ai enriches customer tickets with two types of metadata:

Sentiment tags, which describe the emotional tone of the message
Channel tags, which describe how the message was received

These two types were combined using an intersection to create the TicketMetadata type – but there's a bug! Hover over TicketMetadata and notice that it's never due to an incompatible intersection.

Update the TicketMetadata to hold both the SentimentTag and ChannelTag types inside it as properties. Call them sentiment and channel respectively.
Update the getReviewMethod function to work with your new nested structure. It should have the same logic as before:

getReviewMethod should return "manual_review" if any of the following are true:

The sentiment is flagged
The channel is not verified
The channel type is "phone"

Otherwise, it should return "auto_process".