VS Code Editor
Building and maintaining 2.9 million lines of TypeScript powering the world's most popular code editor. Complex plugin architecture, performance-critical rendering, and cross-platform compatibility.
Payment Dashboard & APIs
Type-safe payment infrastructure processing billions in transactions. Typed API clients, webhook systems with typed event payloads, and complex financial data visualization.
Frontend Migration & Booking Platform
Migrated 2 million lines from JavaScript to TypeScript. Type-safe booking flows, search interfaces with typed filter state, and host management tools.
Collaborative Design Tool
Real-time multiplayer editing with typed CRDT operations, complex canvas rendering with typed graphics APIs, and a plugin system with strict type boundaries.
What TypeScript Developers Actually Build
Before you write your job description, understand what a TypeScript developer will do at your company. Here are real examples from industry leaders:
Enterprise Applications & Developer Tools
Microsoft builds VS Code entirely in TypeScript—2.9 million lines of type-safe code that millions of developers use daily. Their TypeScript developers handle:
- Complex plugin architectures with strict type contracts
- Performance-critical rendering with typed state management
- Cross-platform compatibility (Electron, web, remote)
Figma uses TypeScript for their collaborative design tool:
- Real-time multiplayer editing with typed CRDT operations
- Complex canvas rendering with typed graphics APIs
- Plugin system with strict type boundaries
Payment Systems & Fintech
Stripe builds their entire payment infrastructure with TypeScript. Their developers create:
- Type-safe API clients that prevent integration errors
- Dashboard interfaces handling billions in transaction data
- Webhook systems with typed event payloads
Plaid uses TypeScript for financial data aggregation:
- Type-safe bank integrations across 12,000+ institutions
- Complex data transformation pipelines
- Compliance-critical audit logging
Consumer Platforms
Airbnb migrated 2 million lines from JavaScript to TypeScript:
- Booking flows with typed form validation
- Search interfaces with typed filter state
- Host tools with typed listing management
Slack built their desktop app with TypeScript:
- Real-time messaging with typed WebSocket events
- Complex notification systems
- Cross-platform electron architecture
TypeScript vs JavaScript: What Changed
Understanding this difference helps you evaluate candidates. JavaScript developers who "also know TypeScript" are very different from developers who think in types.
The JavaScript Mindset
JavaScript developers write code and find bugs in testing (or production):
function processUser(user) {
return user.name.toUpperCase(); // Crashes if user is null
}
The TypeScript Mindset
TypeScript developers design types first, then implementation:
interface User {
id: string;
name: string;
email: string | null;
}
function processUser(user: User): string {
return user.name.toUpperCase(); // TypeScript knows this is safe
}
The difference isn't syntax—it's thinking. A JavaScript developer adds types to make errors go away. A TypeScript developer designs types to make errors impossible.
The Modern TypeScript Developer (2024-2026)
TypeScript has evolved dramatically. The ecosystem in 2026 looks very different from 2020.
Beyond Basic Types
If a candidate only knows string, number, and boolean, they're at the tutorial level. Modern TypeScript uses:
- Union types:
status: "pending" | "success" | "error"instead ofstatus: string - Discriminated unions: Type-safe pattern matching for complex states
- Template literal types:
EventName<"click" | "hover">generates"onClick" | "onHover"
Type Inference Mastery
Juniors annotate everything. Seniors let TypeScript infer:
// Junior: Redundant annotations everywhere
const name: string = "hello";
const numbers: number[] = [1, 2, 3];
// Senior: Let TypeScript work
const name = "hello"; // TypeScript knows it's string
const numbers = [1, 2, 3]; // TypeScript knows it's number[]
Runtime Validation
Types only exist at compile time. Senior developers understand the boundary:
// API responses need runtime validation
import { z } from 'zod';
const UserSchema = z.object({
id: z.string(),
name: z.string(),
email: z.string().email()
});
type User = z.infer<typeof UserSchema>; // Type derived from runtime schema
Companies like Stripe and Airbnb require this pattern for API boundaries.
Recruiter's Cheat Sheet: Spotting Great Candidates
Conversation Starters That Reveal Skill Level
Instead of asking "Do you know TypeScript?", try these:
| Question | Junior Answer | Senior Answer |
|---|---|---|
"When do you use any vs unknown?" |
"They're similar" | "Never any in production. Unknown requires type guards before use—safer for API responses" |
| "How do you type a function that works with any array?" | "Use any[]" | "Generics: `function first(arr: T[]): T |
| "What's your tsconfig strictness?" | "Whatever the default is" | "strict: true, noUncheckedIndexedAccess, exactOptionalPropertyTypes" |
Resume Signals That Matter
✅ Look for:
- Mentions TypeScript in production contexts, not just tutorials
- Experience with strict mode enabled
- Contributions to DefinitelyTyped or typed libraries
- Specific projects: "Migrated 50K lines to TypeScript" or "Built type-safe API client"
🚫 Be skeptical of:
- "TypeScript expert" with only 6 months experience
- No mention of type safety in project descriptions
- Only used TypeScript because the framework required it
GitHub Portfolio Red Flags
anytypes scattered throughout code// @ts-ignorecomments everywhere- tsconfig.json with strict mode disabled
- No type declarations in function parameters
- Types in separate
.d.tsfiles when inline types would work
Skills to Look For: By Experience Level
Junior TypeScript Developer
Must demonstrate:
- Basic types: string, number, boolean, arrays, objects
- Interfaces and type aliases
- Function typing (parameters and return types)
- Understanding of null/undefined handling
Ask: "Show me how you'd type a function that fetches user data."
Mid-Level TypeScript Developer
Must demonstrate:
- Generics for reusable code
- Union types and type guards
- Utility types (Pick, Omit, Partial, Required)
- Experience with strict mode
Ask: "How would you type a function that can accept any object with an 'id' field?"
Senior TypeScript Developer
Must demonstrate:
- Conditional types and mapped types
- Template literal types
- Type inference optimization
- Migration strategies for large codebases
- Runtime validation patterns
Ask: "Walk me through migrating a 100K-line JavaScript codebase to TypeScript."
Common Hiring Mistakes
1. Treating TypeScript as Separate from JavaScript
TypeScript IS JavaScript with types. A candidate excellent at JavaScript but new to TypeScript will be productive in days. The reverse isn't true—someone who learned TypeScript without understanding JavaScript fundamentals will struggle with debugging, performance, and the DOM.
Better approach: Assess JavaScript depth first. A strong JS developer learns TypeScript quickly.
2. Overvaluing Advanced Type Gymnastics
Complex conditional types impress in interviews but rarely matter in application code. Stripe's codebase is mostly straightforward types—the complexity is in the business logic, not the type system.
Better approach: Focus on whether candidates write maintainable, well-typed code, not whether they can implement a type-level parser.
3. Ignoring the Migration Story
Most codebases aren't pure TypeScript. Ask about:
- Experience migrating JavaScript to TypeScript
- Working with untyped libraries
- Gradual adoption strategies (allowJs, checkJs)
- Typing third-party packages without types
4. Not Testing TypeScript Configuration
Ask candidates about tsconfig.json settings. Developers who work with strict mode enabled write better code. Key settings:
strict: true- The baseline for professional TypeScriptnoImplicitAny- Catches untyped codestrictNullChecks- Prevents null reference errorsnoUncheckedIndexedAccess- Safer array/object access
If they don't know these settings, they've probably only worked in permissive codebases.