Summary

In this lesson, you learned that:

TypeScript works out the type of an object from its properties: { name: string; health: number; }. You can't add a property that isn't in the type (Property 'gold' does not exist on type ...), so give an object all its properties when you create it.
An object type can be written as an annotation, let target: { name: string; health: number };, or named with a type alias: type Hero = { name: string; health: number };. Then use Hero for variables, parameters, return types and arrays (Hero[]).
In an object literal, a missing property gives TS2741 (Property 'price' is missing in type ...), an extra one TS2353, and a typo TS2561 with Did you mean to write 'price'?.
An optional property has a question mark, nickname?: string. It may be missing, its type is string | undefined, and it must be checked before use.
A readonly property can't be changed after the object is created: Cannot assign to 'name' because it is a read-only property. Unlike const, it protects one property, not a variable.
An interface describes an object too: interface Monster { name: string; health: number }. interface Boss extends Monster { phase: number } adds properties, and a Boss can be used wherever a Monster is needed.
A property can hold another object: position: Position. Inner types with their own names make the types and error messages shorter.
With Item[], every element of the array is checked, and in for...of, every item is an Item.
In functions, object types are parameter types (function train(monster: Monster): void) and return types (function createMonster(name: string): Monster). A function that changes a property changes the very object you passed to it, not a copy.

In the next lesson, Unions and Narrowing, you'll meet types that allow several kinds of values, such as string | number, or a target that can be null, and you'll see how TypeScript follows your conditions to know which one you have.

Instructions

Let's practice. Create an interface Quest with a read-only property id (a number), a title (a string), a reward (a number) and an optional note (a string).

Create an array quests of type Quest[] with three quests: id 1, “Wolves in the Woods”, 50 gold; id 2, “The Lost Crown”, 120 gold, with the note "ask the miller"; id 3, “The Goblin Bridge”, 30 gold. Write a function describeQuest that takes a Quest and returns its line; when the quest has a note, the note goes at the end, in parentheses. With a for...of loop, print every quest:

#1 Wolves in the Woods: 50 gold
#2 The Lost Crown: 120 gold (ask the miller)
#3 The Goblin Bridge: 30 gold

Write a function totalReward that takes a Quest[] and returns the sum of the rewards. Print:

Total reward: 200 gold

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