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Optional Parameters

You can specify function parameters as optional with a question mark (?) after the name:

function greet(name: string, title?: string): string {
  if (title) {
    return `Hello, ${title} ${name}!`;
  }
  return `Hello, ${name}!`;
}

greet("Gandalf");           // "Hello, Gandalf!"
greet("Gandalf", "Wizard"); // "Hello, Wizard Gandalf!"

There are two rules to keep in mind:

Optional parameters must come after all required parameters. For example, this code won't compile:

// Error: Required parameter cannot follow optional parameter
function greet(title?: string, name: string): string {
  // ...
}

Optional params have an undefined automatically unioned on the specified type. If the value is omitted, it's undefined instead of the specified type.

function greet(name: string, title?: string): string {
  // inside the function, title
  // is a string | undefined
}

Assignment

At Support.ai, we're building a service that calculates pricing for our API usage. Different tiers of customers get different rates.

Modify the calculateApiCost function signature. The tier parameter should be optional.
Complete the function. It should return the cost of API calls based on the number of requests and (optionally) the tier.
- If no tier is provided, or if the tier is not recognized, each request costs 0.10
- If the pro tier is specified, each request costs 0.05
- If the enterprise tier is specified, each request costs 0.03