Types for variables
In TypeScript you add types after a colon: let name: string = 'Alex'. The editor and compiler will warn you if you use the wrong type. Here we write valid JS that would be valid TS too (types in comments).
Appy Says…
TypeScript adds type safety to JavaScript — it catches bugs before you run the code. That's how teams at Google, Airbnb, and Spotify write millions of lines without entire categories of runtime errors.
What are TypeScript Simple Types?
TypeScript lets you annotate variables, parameters, and return values with a type. The compiler checks that you're using the right types everywhere.
- •
let name: string = 'Appy'; - •
let age: number = 14; - •
let isAdmin: boolean = false; - •
let score: number;— declared, initially undefined - •Function:
function greet(name: string): string { return 'Hi ' + name; } - •Type inference:
let x = 42;→ TS infersx: numberautomatically - •Explicit annotation only needed when inference can't determine the type
Think of it like Minecraft inventory slots
A sword slot only accepts swords — you can't put a carrot there. TypeScript is this slot system for your code. A function expecting a string won't accept a number — TypeScript tells you before the game even starts.
How It Works
- •1. TypeScript is a superset of JavaScript — valid JS is valid TS
- •2. Annotate: add
: Typeafter variable names and parameters - •3. The
tsccompiler checks types and outputs plain JavaScript - •4. Errors show in your editor immediately (red squiggles) before running
- •5. No runtime overhead — types are erased at compile time
- •6. Strict mode (
"strict": truein tsconfig) enables the most useful checks
Real-World Examples
- •
function getUser(id: number): User { ... } - •Passing a string where a number is expected → red squiggle immediately
- •React component:
function Button({ label }: { label: string }) { ... } - •All of Applaa's codebase uses TypeScript — it caught dozens of bugs before they shipped
Key Facts
- •TypeScript was created by Microsoft (Anders Hejlsberg) and open-sourced in 2012
- •Stack Overflow 2024 survey: TypeScript is the 5th most popular language overall
- •Types exist only at compile time — they're completely erased in the output JS
- •TypeScript can infer most types — you don't need to annotate everything
Watch Out!
Don't overuse any type — it disables all type checking for that value, defeating the purpose of TypeScript. If you're not sure of the type, use unknown instead — it forces you to check before using the value.
Remember
TypeScript catches type errors at compile time, not runtime. Let inference do the work — only annotate when TS can't figure it out. Avoid any.
What You Learned
- •TypeScript adds type annotations:
variable: type,param: type,): returnType - •Types are compile-time only — erased in output JS; TS infers most types automatically
- •Unlocks: catching bugs before runtime, editor autocomplete, safer refactoring
Key Facts
- →TypeScript was created by Microsoft (Anders Hejlsberg) and open-sourced in 2012
- →Stack Overflow 2024 survey: TypeScript is the 5th most popular language overall
- →Types exist only at compile time — they're completely erased in the output JS
- →TypeScript can infer most types — you don't need to annotate everything
Real-World Examples
Remember
TypeScript catches type errors at compile time, not runtime. Let inference do the work — only annotate when TS can't figure it out. Avoid any.
Quick Quiz
What does TypeScript add to JavaScript?