TypeScript Advanced

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Last Updated: May 2, 2026

Advanced TypeScript: conditional types, template literal types, mapped types, infer keyword, discriminated unions, and declaration files.

Conditional Types

Pattern	Type Expression
Basic conditional	`T extends U ? X : Y`
Distributive	`T extends unknown ? ... : never`
Exclude null	`type NonNull<T> = T extends null \| undefined ? never : T`
`infer`	`T extends Promise<infer U> ? U : never`

`${Prefix}${string}`

Match strings starting with a prefix for event names, CSS classes.

Uppercase / Capitalize

Transform literals: Capitalize<'hello'> yields 'Hello'.

Utility	Definition
`Partial<T>`	`{ [K in keyof T]?: T[K] }`
`Pick<T, K>`	`{ [P in K]: T[P] }`
Key remapping	{ [K in keyof T as `get${Capitalize<K>}`]: () => T[K] }

kind field

if (shape.kind === 'circle') narrows type automatically.

Exhaustiveness

default: const _: never = shape;

Pro Tip: Use never type in default case of discriminated union switches for compile-time exhaustiveness checking.