Objects in Functions

Object types are most useful in functions, as parameter types and return types. The first line of a function then tells the whole story: what the function takes and what it gives back.

A function that creates objects of one shape, often called a factory, gets the type as its return type. TypeScript then checks the object it returns:

interface Monster {
    name: string;
    level: number;
    health: number;
}

function createMonster(name: string, level: number): Monster {
    return { name: name, health: level * 10 };
}

main.ts(8,5): error TS2741: Property 'level' is missing in type '{ name: string; health: number; }' but required in type 'Monster'.

Without the return type, the function would return a monster without a level, and the mistake would show up only later, wherever a monster's level is read. With it, TypeScript reports the return itself.

Here is the working version, together with a function that changes a monster:

interface Monster {
    name: string;
    level: number;
    health: number;
}

function createMonster(name: string, level: number): Monster {
    return { name: name, level: level, health: level * 10 };
}

function enrage(monster: Monster): void {
    monster.level = monster.level + 2;
}

const wolf = createMonster("wolf", 3);
enrage(wolf);
console.log(wolf);

This prints:

{ name: "wolf", level: 5, health: 30 }

wolf gets the type Monster from the function's return type, without an annotation. enrage changes the very object that you pass to it, not a copy, so the change is visible outside the function, just as levelUp changed Kira's level in the exercise on read-only properties. Its return type is void: it doesn't return anything, the change is its result.

The calls are checked as with any other parameter type:

interface Monster {
    name: string;
    level: number;
    health: number;
}

function enrage(monster: Monster): void {
    monster.level = monster.level + 2;
}

enrage("wolf");
enrage({ name: "troll", level: 4, health: 40, angry: true });

main.ts(11,8): error TS2345: Argument of type 'string' is not assignable to parameter of type 'Monster'.
main.ts(12,47): error TS2353: Object literal may only specify known properties, and 'angry' does not exist in type 'Monster'.

The first call passes a text instead of a monster. The second one passes an object literal with an extra property, as in the exercise on extra properties.

Typical mistakes:

• A property missing from the returned object: Property 'level' is missing in type ..., reported at the return.
• Passing something other than the object, such as the monster's name: Argument of type 'string' is not assignable to parameter of type 'Monster'.