What is the result of this conditional type?
type IsArray<T> = T extends any[] ? 'array' : 'not array';
type A = IsArray<string[]>;
type B = IsArray<number>;What does this mapped type produce?
type Nullable<T> = { [K in keyof T]: T[K] | null };
type User = { name: string; age: number };
type NullableUser = Nullable<User>;What does the 'infer' keyword do in conditional types?
type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;
type Result = ReturnType<() => { id: number; name: string }>;What is a discriminated union and what makes it work?
type Shape =
| { kind: 'circle'; radius: number }
| { kind: 'rect'; width: number; height: number };
function area(s: Shape) {
switch (s.kind) {
case 'circle': return Math.PI * s.radius ** 2;
case 'rect': return s.width * s.height;
}
}What does declaration merging allow in TypeScript?
interface Window {
myPlugin: { version: string };
}
// Now window.myPlugin is typed!
declare const window: Window;
window.myPlugin.version; // no errorWhat does the 'satisfies' operator do in TypeScript 4.9+?
const palette = {
red: [255, 0, 0],
green: '#00ff00',
} satisfies Record<string, string | number[]>;
palette.red.map(x => x); // works! red is inferred as number[]
palette.green.toUpperCase(); // works! green is inferred as stringWhat is the difference between Partial<T>, Required<T>, and Pick<T, K>?
What does this template literal type produce?
type EventName = 'click' | 'focus' | 'blur';
type Handler = `on${Capitalize<EventName>}`;