Objects, interfaces and readonly

Goal

Model Streak's data with interfaces, optional fields and readonly.

An interface is a shape

From streak/src/types.ts:

export interface Habit {
  readonly id: string
  readonly name: string
  readonly emoji: string
  readonly frequency: Frequency
  readonly createdOn: Day
  readonly checkIns: readonly Day[]
}

Any object with those fields is a Habit. TypeScript checks shapes, not class names.

Optional fields

What someone gives us to create a habit has fewer fields, and some are optional:

export interface NewHabit {
  name: string
  emoji?: string
  frequency?: Frequency
}

emoji?: string means "a string, or not there at all". When you read it you get string | undefined, so you must handle the missing case. addHabit does it with ??:

emoji: input.emoji ?? '✅',

With exactOptionalPropertyTypes, "not there" and "there but undefined" are different, which is why the CLI spreads optional fields in only when they exist:

{ name: rest.join(' '), ...(emoji ? { emoji } : {}), ...(frequency ? { frequency } : {}) }

readonly: change by copying

readonly stops code from changing a value in place. To "change" a habit, Streak makes a new one. From checkIn in streak/src/habits.ts:

return ok({ ...habit, checkIns: [...habit.checkIns, day].sort() })

The old habit is untouched. Nothing else that holds it is surprised, and tests stay simple.

type or interface?

Both describe object shapes. A common, simple rule: interface for object shapes, type for everything else (unions, literal sets, functions). Streak follows it.

Check it works

In checkIn, add habit.checkIns.push(day). The checker refuses with Property 'push' does not exist on type 'readonly `${number}-${number}-${number}`[]'. Undo it.

Exercise

Add an optional note?: string to NewHabit and Habit, and make addHabit copy it across only when it is given. Run npx tsc -p streak --noEmit until it is clean.