TypeScript Basics

TypeScript is a static type-checker for JavaScript. It identifies potential errors in code before it runs, providing a more robust development experience.

The Role of Static Type-Checking

JavaScript only provides dynamic typing, where errors are discovered at runtime. TypeScript adds a static type system that makes predictions about code behavior based on the shapes and capabilities of values.

Non-Exception Failures

TypeScript catches "valid" JavaScript that is likely a bug, such as:

Accessing non-existent properties (user.location where location isn't defined).
Typos in property or method names.
Uncalled functions (e.g., Math.random < 0.5).
Basic logic errors (unreachable code due to impossible conditions).

Tooling and `tsc`

The TypeScript compiler (tsc) transforms .ts files into plain .js files.

Type Erasure: Type annotations are completely removed during compilation; they do not exist at runtime.
Downleveling: TypeScript can transpile modern JavaScript features (like template strings) to older versions (like ES5) for better compatibility.

Strictness Settings

TypeScript's strictness can be adjusted to suit the project's needs. Key flags include:

noImplicitAny: Issues an error when a variable's type is implicitly inferred as any.
strictNullChecks: Makes handling null and undefined explicit, preventing "billion dollar mistake" bugs.
strict: Enables all strict type-checking options simultaneously.

References