as is not a castPlease, do not use the word “cast” to describe the as keyword!
That’s far from what it actually does.
Casting is usually used to describe the coercion or mapping from one type to another.
Here is an example of casting in Java:
double d = 1.23d;
int i = (int) d; // warning: lossy coercion!
We’re simply casting a value of type double to type int, using (int) in front of the value.
At runtime, this really does convert the value of 1.23d to an int (1), but in TypeScript, this is not the case.
Given the following, similar code;
const str = "1.23";
const num = str as number;
console.log(num);
The result is actually "1.23".
TypeScript does not (and will not — please refer to the non-goals) add runtime type casting.
The key here is in the name!
It’s called an assertion.
Still doubting?
Refer to the official documentation on the subject.
Use as sparingly, as it is basically lying to the compiler.
Only assert things if you are 100% sure.
This goes the same for ! (non-null assertion), which should be used in the case where you just want to remove null/undefined from the type:
const a = null!; // never
const b = localStorage.getItem("key"); // string | null
const c = localStorage.getItem("key")!; // string
Originally, the syntax for assertions was different.
However, since it was so similar to casting in other languages, people started to confuse it for casting at runtime!
That is one of the reasons why we have this new as keyword to describe assertions.
See for yourself:
const str = "1.23";
const num = <number> str;
Chilling!