as keyword

Learn type assertion with as.

What is the as keyword?

The as keyword is used for type assertion in TypeScript. You tell the compiler that a value is of a specific type; this does not change the value at runtime, it is only compile-time information. It is commonly used to narrow any or unknown values to a more specific type.

let value: any = "hello";
const length = (value as string).length;

When to use as?

as is typically used to explicitly state a type the compiler cannot infer. For example, when you want to treat a DOM element as an HTMLInputElement. It is also common for safely casting an unknown value to a specific type.

const input = document.getElementById("name") as HTMLInputElement;
console.log(input.value);

Exercise String length with as

The deger variable is of type any with value "merhaba". Use as to assert it as string and print its .length property with console.log.

Show example solution

const deger: any = "merhaba";
const metin = deger as string;
console.log(metin.length);

Test yourself

What is the runtime effect of the as keyword?
- No effect, works only at compile time
- Converts the value to a new type
- Permanently changes the variable's type
- Creates a new variable
Show answer

No effect, works only at compile time — as is compile-time only; it does not change the value at runtime at all.

Assert as string with as

const deger: any = "merhaba";
const metin = deger ____ string;
console.log(metin.length);

Show answer

const deger: any = "merhaba";
const metin = deger as string;
console.log(metin.length);

as usage

as

Type assertion keyword
derleme zamanı

Assertion works here
çalışma zamanı

Value unchanged
any

Unsafe base type

Next lesson: angle bracket syntax

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