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Type Predicates

Sometimes the built-in type guards (typeof, instanceof, etc.) aren't enough.

TypeScript allows you to create your own type guards using type predicates. We do that by creating a function that:

Accepts a wide type that we want to narrow
Returns a boolean indicating if the value is of the desired type
Uses the type predicate syntax value is Type in the return type

For example, here's a function that reports if a value is a string:

function isString(value: unknown): value is string {
  return typeof value === "string";
}

function processValue(value: unknown) {
  if (isString(value)) {
    // TypeScript knows value is a string here
    console.log(value.toUpperCase());
  }
}

For simple stuff like this, we could have just inlined the typeof check:

function processValue(value: unknown) {
  if (typeof value === "string") {
    // TypeScript knows value is a string here
    console.log(value.toUpperCase());
  }
}

But type predicates become really useful when the logic to check the type is a bit more complex. So we have a situation where one type, in this case, the ManagerAdmin type, shares properties with both other types:

interface ManagerAdmin {
  accessLevel: number;
  numEmployees: number;
}

interface Admin {
  accessLevel: number;
  payrollDate: Date;
}

interface Manager {
  numEmployees: number;
}

We can encapsulate the slightly more complex logic in a type predicate function:

function isManagerAdmin(
  boss: ManagerAdmin | Admin | Manager,
): boss is ManagerAdmin {
  return "numEmployees" in boss && "accessLevel" in boss;
}

// boss is a `ManagerAdmin | Admin | Manager`
if (isManagerAdmin(boss)) {
  // TypeScript knows boss is a ManagerAdmin here
  console.log(`Managing ${boss.numEmployees} employees`);
}

Assignment

We support multiple LLMs, those per-token prices aren't gonna arbitrage themselves...

Anyhow, we need to write code to activate a specific model when starting a chat with a customer.

Fix isModelSkippity: it should be a type predicate function.
Fix isModelJean by also making it a type predicate function.
Complete the activateModel function:
1. If the given model is Skippity, return the string Activated model Skippity version {VERSION} with searching {STATUS}
  - {VERSION} is simply the version property
  - {STATUS} is either enabled or disabled based on the search property
2. If the given model is Jean, return the string Activated model Jean version {VERSION} with thinking {STATUS}
  - {VERSION} is simply the version property
  - {STATUS} is either enabled or disabled based on the think property