Type Declaration vs Type Assertions

Simple answer, you should prefer Type Declaration to Type Assertion, why? read along…

There are two ways of providing type to a variable in typescript

1. Type Declaration
2. Type Assertion

Let’s see what is the difference between these two methods and when to use which method. In the below code we are defining an interface called Hero which will be used to provide type to the following variables. We are using Type Declaration to assign a type to the variable drowand Type Assertion to assign a type to the variable centaur.

interface Hero { 
  name: string, 
  type: string 
};

// Type Declaration
const drow: Hero = {
  name: 'Drow Ranger',
  type: 'agility'
};

// Type Assertion
const centaur = {
  name: 'centaur warrunner',
  type: 'strength'
} as Hero;

In a perfect world, like in the example above, both the objects have the expected fields and there is no difference between both types of type assignment. Let’s see how these works in the following scenarios.

Type Declaration

Type Declaration verifies that the value conforms to the interface. Since the following examples do not abide by it, Typescript throws an error.

// Error: Properties missing in the type
const drow: Hero = {};

// Error: 'isAvailable' does not exist on type Hero
const drow: Hero = {
  name: 'centaur warrunner',
  type: 'strength',
  isAvailable: true,
};

Type Assertion

Type Assertion tells the type checker that you understand the types better than it and to not throw an error.

// No Error
const drow = {} as Hero;

// No Error
const drow = {
  name: 'centaur warrunner',
  type: 'strength',
  isAvailable: true,
} as Hero;

So, in conclusion, you should use Type Declaration over Type Assertion because they provide additional safety checks. Unless you have a specific use case to use Type Assertion

Thats all folks, happy hacking 🙌