What sets apart using `: Interface` versus `as Interface` in Typescript?

I'm struggling to find the distinction between two similar lines of code due to uncertainty about what they are called. Consider the scenario where the following interface is defined:

interface Person {
  name: string;
  age: number;
}

What exactly separates the following pieces of code?

const foo: Person = getPerson(data);

and 

const foo = getPerson(data) as Person;

It appears that both lines inform TypeScript that foo is a Person. Is there a significant difference between the two, or is it simply a matter of personal preference?

javascripttypescript

Answer №1

When you use

const foo: Person = getPerson(data);

you are essentially informing the compiler: I am assuming that the function getPerson will return an object that follows the Person interface. Please verify this and notify me if my assumption is incorrect.

const foo = getPerson(data) as Person;

On the other hand, this statement is an assertion, where you explicitly declare to the compiler that the output of getPerson will be a Person, even if this is not guaranteed. In essence, it's a way to bypass the type checker, although it is generally not recommended.

Answer №2

let person: Person = getPerson(data);

When you use this type annotation, you are essentially specifying that you want the variable person to be of type Person. If the function getPerson(data) returns something other than a Person, TypeScript will give an error stating that the returned type does not match the expected Person.

const person = getPerson(data) as Person;

The as Person in this code is a type assertion. Without it, TypeScript would infer the type of person based on whatever type getPerson(data) returns. However, by adding as Person, you are explicitly telling TypeScript to treat the result as a Person regardless of its actual type.

I hope this explanation clarifies things for you.

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