15. Type assertions

Type Assertions allow you to tell TypeScript to treat a variable as a specific type, overriding its inferred type. It’s similar to type casting in other languages, but without any special checking or restructuring.

Example 1: Basic Type Assertion

let someValue: any = "this is a string";
let strLength: number = (<string>someValue).length;

• Here, someValue is of type any, but we know it’s a string, so we assert it as a string using <string>someValue.
• Then we access the length property, which TypeScript knows is valid for a string.

Example 2: Using as Syntax (alternative to <string>)

let strLength: number = (someValue as string).length;

• The as syntax is the alternative to the <type> syntax and is preferred in JSX/TSX files (for React).

Key Points:

• Type assertions are used to tell TypeScript the specific type of a variable when TypeScript cannot infer it automatically.
• <type>value and value as type are two ways to write type assertions.
• Type assertions do not perform any runtime checks; they just tell TypeScript to treat the value as a specified type.