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TypeScript Academy · Lesson

Generic Constraints: Narrowing Type Parameters

Use extends to enforce shape requirements on generics.

Lesson 2 of 413 steps

Generic Constraints: Narrowing Type Parameters is a free TypeScript Academy lesson on CoddyKit. This is lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 4 lessons in total.

Welcome

Advanced generic constraints let you narrow what types can be used as type arguments, enabling type-safe operations on complex shapes.

Constraint Recap

T extends U restricts T to types that are subtypes of U. This gives TypeScript knowledge about T's shape inside the function.
function len<T extends { length: number }>(val: T): number {
  return val.length;
}

Constraining to Specific Methods

Require T to have a specific method by constraining to an interface with that method.
interface Stringable { toString(): string; }
function stringify<T extends Stringable>(val: T): string {
  return val.toString();
}

Infer from Constraint

Use keyof within the constraint to build safe accessors.
function pluck<T, K extends keyof T>(items: T[], key: K): T[K][] {
  return items.map(item => item[key]);
}
const names = pluck(users, 'name'); // string[]

Higher-Kinded Constraints (Simulated)

TypeScript doesn't support true higher-kinded types, but you can simulate them with conditional types and constraints.

Constraining to Primitive Types

Restrict type parameters to specific primitive types for specialized functions.
type Primitive = string | number | boolean | bigint | symbol | null | undefined;
function isPrimitive<T extends Primitive>(val: T): true { return true; }

Recursive Constraints

Constraints can reference themselves for recursive patterns.
interface Comparable<T> { compareTo(other: T): number; }
function sort<T extends Comparable<T>>(items: T[]): T[] {
  return [...items].sort((a, b) => a.compareTo(b));
}

Constraint with Default

A type parameter can have both a constraint and a default. The default must satisfy the constraint.
function process<T extends object = Record<string, unknown>>(data: T): void {
  console.log(Object.keys(data));
}

T extends Record<string, unknown>

Constrain to a record type to enable dynamic property access.
function getValue<T extends Record<string, unknown>>(obj: T, key: string): unknown {
  return obj[key];
}

Constraint Inference in Return Types

When T is constrained, TypeScript uses the constraint to determine valid operations on the return type.
function firstKey<T extends object>(obj: T): keyof T {
  return Object.keys(obj)[0] as keyof T;
}

NoInfer Utility (TS 5.4)

TypeScript 5.4 introduced NoInfer to prevent a type parameter from being inferred from a specific argument position.
function createState<T>(initial: T, fallback: NoInfer<T>): T {
  return initial ?? fallback;
}

Quick Check

What does `T extends { length: number }` add to T inside a generic function?
Answer it in the interactive lesson

Recap

Advanced constraints narrow generic types to specific shapes, methods, or primitive unions. Combine with keyof, recursive patterns, and defaults to build powerful type-safe utilities.
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Frequently Asked Questions

Is the “Generic Constraints: Narrowing Type Parameters” lesson free?

Yes — the full text of “Generic Constraints: Narrowing Type Parameters” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 4 lessons in total.

What will I learn in “Generic Constraints: Narrowing Type Parameters”?

Use extends to enforce shape requirements on generics. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start TypeScript Academy?

No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

How long does the “Generic Constraints: Narrowing Type Parameters” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this TypeScript Academy lesson?

Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. keyof and Indexed Access Types
  2. Generic Constraints: Narrowing Type Parameters
  3. Conditional Types: T extends U ? X : Y
  4. Distributive Conditional Types
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