The Compiler & Strictness2026-04-02

TypeScript helps catch mistakes before the code runs. It gives you static type checking during compilation, while JavaScript checks types at runtime. You also get useful suggestions while typing.

The Compiler (tsc)

tsc is the TypeScript compiler.

Install it globally:

npm install -g typescript

Compile a .ts file into JavaScript:

tsc file_name.ts

This generates a file_name.js file.

Strictness Flags

TypeScript has strictness settings in your tsconfig.json that help keep the codebase safer.

noImplicitAny makes sure TypeScript does not silently allow implicit any types.

strictNullChecks helps catch null and undefined issues before runtime.

Type Erasure

TypeScript types are completely removed during compilation. The runtime only sees plain JavaScript, so types have zero impact on runtime performance.

Downleveling

tsc can translate modern ES6+ JavaScript into older JavaScript targets. This helps support legacy browsers while still letting you write modern code.