Christopher Tran pfftdammitchris

💭

Dreaming

JavaScript Developer. React, NodeJS, TypeScript

pfftdammitchris / snippet-2.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-2.ts

	function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
	return obj[key];
	}

	const user = { id: 42, name: "Alice" };

	// Infers return type as number
	const userId = getProperty(user, "id");

	// Infers return type as string

pfftdammitchris / snippet-3.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-3.ts

	function updateProperty<T, K extends keyof T>(
	obj: T,
	key: K,
	value: T[K]
	): T {
	return { ...obj, [key]: value };
	}

	interface Config {
	port: number;

pfftdammitchris / snippet-4.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-4.ts

	type PathsToStringProps<T> = {
	[K in keyof T]: T[K] extends string
	? K
	: T[K] extends object
	? `${K & string}.${PathsToStringProps<T[K]> & string}`
	: never;
	}[keyof T];

	function getNestedString<T, P extends PathsToStringProps<T>>(
	obj: T,

pfftdammitchris / snippet-5.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-5.ts

	type IsString<T> = T extends string ? true : false;

	type A = IsString<"hello">; // true
	type B = IsString<42>; // false

	type ExtractStrings<T> = T extends string ? T : never;

	type C = ExtractStrings<"a" \| "b" \| 42 \| true>; // "a" \| "b"

pfftdammitchris / snippet-6.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-6.ts

	type Nullable<T> = {
	[K in keyof T]: T[K] \| null;
	};

	type Optional<T> = {
	[K in keyof T]?: T[K];
	};

	type DeepReadonly<T> = {
	readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K];

pfftdammitchris / snippet-7.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-7.ts

	type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

	function getUser(): { id: number; name: string } {
	return { id: 1, name: "Alice" };
	}

	type User = ReturnType<typeof getUser>;
	// { id: number; name: string }

	type FirstArg<T> = T extends (arg: infer A, ...rest: any[]) => any ? A : never;

pfftdammitchris / snippet-8.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-8.ts

	type RequireKeys<T, K extends keyof T> = T & Required<Pick<T, K>>;

	interface Config {
	host?: string;
	port?: number;
	debug?: boolean;
	}

	type ProductionConfig = RequireKeys<Config, "host" \| "port">;
	// { host: string; port: number; debug?: boolean }

pfftdammitchris / snippet-9.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-9.ts

	type RequiredKeys<T> = {
	[K in keyof T]-?: {} extends Pick<T, K> ? never : K;
	}[keyof T];

	function getRequiredProperty<T, K extends RequiredKeys<T>>(
	obj: T,
	key: K
	): T[K] {
	return obj[key]; // Guaranteed to be defined
	}

pfftdammitchris / snippet-10.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-10.ts

	// Overconstrained: requires full User type
	function logUserId<T extends User>(user: T): void {
	console.log(user.id);
	}

	// Correctly constrained: requires only id property
	function logId<T extends { id: number }>(obj: T): void {
	console.log(obj.id);
	}

pfftdammitchris / snippet-11.ts

Created July 18, 2026 06:06

TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-11.ts

	interface DatabaseConfig {
	host: string;
	port: number;
	ssl: boolean;
	poolSize: number;
	}

	const defaults: Partial<DatabaseConfig> = {
	ssl: true,
	poolSize: 10,