mediumtypeof type queries
What does `BoxCtor` represent, and which expression below is therefore valid?
class Box {
value = 0;
static make() { return new Box(); }
}
type BoxCtor = typeof Box;TypeScript Interview Questions (606 to practise)
TypeScript adds a structural type system on top of JavaScript. Most of the difficulty sits in what the compiler infers, not in what you wrote.
Expect inference puzzles: conditional types, mapped types, variance on function parameters, and when a union distributes. A large share of questions are literally 'what is the inferred type here'.
- Total questions
- 606
- Easy
- 125
- Medium
- 219
- Hard
- 262
Where it shows up: Product engineering teams that outgrew plain JavaScript, design-system work, and any codebase large enough to need refactor safety.
Sample TypeScript questions
Twenty real TypeScript questions from the library, code snippet included. Nothing here is paraphrased for search engines. It is the same text a signed-in user sees.
hardarrays and tuples
What is the type of x?
const arr: number[] = [1,2,3];\nconst x = arr[0]; // with noUncheckedIndexedAccesshardtemplate literal types
What is X?
type X = `${Capitalize<"foo">}Bar`;hardtypeof type queries
What is the type of f?
const f = (x: number) => x + 1;hardprimitive types
What is X?
type X = string & number;mediumprimitive types
What is the type of x?
let x = 5;easytype annotations
What does the type annotation on the `point` parameter require a caller to pass?
function distance(point: { x: number; y: number }): number {
return Math.sqrt(point.x ** 2 + point.y ** 2);
}hardconditional-type distribution
What is X?
type X = Exclude<"a"|"b","a"|"b">;hardconditional-type distribution
How does TypeScript handle the `never` member when this distributive conditional is applied to a union containing `never`?
type ToArray<T> = T extends unknown ? T[] : never;
type R = ToArray<string | never>;mediumgeneric constraints
`keyof` over a union of object types returns the keys common to every member. What is `Result`?
type Result = keyof ({ a: 1; b: 2 } | { b: 3; c: 4 });mediumuser-defined type guards
Both functions test the same condition, but only one narrows. Why does `an2.bark()` fail to compile?
interface Animal { type: string }
interface Dog extends Animal { type: "dog"; bark(): void }
function isDog(a: Animal): a is Dog { return a.type === "dog"; }
function looksDog(a: Animal): boolean { return a.type === "dog"; }
declare const an2: Animal;
if (looksDog(an2)) {
an2.bark();
}mediummapped types
A homomorphic mapped type (one written as `[K in keyof T]`) copies existing modifiers. What is true of `Result` here?
type MyPartial<T> = { [K in keyof T]?: T[K] };
type Result = MyPartial<{ readonly a: number; b: string }>;hardkeyof and indexed access
What is the type of x?
const arr = ["a", "b"] as const;\ntype X = typeof arr[number];hardconditional-type distribution
Wrapping the checked type in a single-element tuple changes the result of these two conditionals. Which pair of results is correct?
type Distrib<T> = T extends never ? 1 : 2;
type NoDistrib<T> = [T] extends [never] ? 1 : 2;
type A = Distrib<never>;
type B = NoDistrib<never>;hardconditional-type distribution
Why does `IsTrue<boolean>` resolve to `"yes" | "no"` rather than a single branch?
type IsTrue<T> = T extends true ? "yes" : "no";
type R = IsTrue<boolean>;hardmethod bivariance strictFunctionTypes
Under `--strictFunctionTypes`, why does the assignment `cmp = cmpDog` below compile even though `cmpDog`'s parameters are narrower than `cmp`'s, an assignment that would be rejected for an equivalent function-typed property?
interface Comparer<T> { compare(a: T, b: T): number }
declare let cmp: Comparer<{ name: string }>;
declare let cmpDog: Comparer<{ name: string; bark(): void }>;
cmp = cmpDog;hardconditional-type distribution
A distributive conditional type `type ToArray<T> = T extends unknown ? T[] : never` is instantiated as `ToArray<string | number>`. What is the *resolved* type, and why?
type ToArray<T> = T extends unknown ? T[] : never;
type R = ToArray<string | number>;mediumconditional-type distribution
Which is the inferred type R?
type IsString<T> = T extends string ? true : false;\ntype R = IsString<string | number>;hardunion and literal types
What is the inferred return type of f?
function f(x: boolean) { return x ? 1 : "a"; }mediumconditional-type distribution
Which is the inferred type T?
type T = Extract<"a" | "b", "b">;Sign in to answer
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TypeScript topics covered
44 distinct TypeScript topics are tagged across the library. These are the real topic labels stored on the questions, not a hand-written list.
- type annotations (32)
- hand-rolled utility types (27)
- primitive types (25)
- ESM CJS interop (23)
- conditional-type distribution (19)
- control-flow analysis pitfalls (19)
- generic constraints (18)
- declaration merging (18)
- excess property checks (17)
- mapped types (17)
- union and literal types (16)
- infer (16)
- inference internals (16)
- arrays and tuples (15)
- moduleResolution modes (15)
- keyof and indexed access (15)
- discriminated unions (14)
- structural typing unsoundness (14)
- user-defined type guards (14)
- NoInfer and const type params (14)
- variadic tuple types (13)
- index signatures (13)
- typeof type queries (13)
- polymorphic this (13)
- verbatimModuleSyntax isolatedModules (13)
- satisfies operator (12)
- interface vs type alias (12)
- template literal types (12)
- conditional types (12)
- assertion functions (12)
- function overloads (11)
- recursive types (10)
- variance co contra (10)
- recursive depth limits (9)
- type-level string parsing (9)
- branded nominal typing (9)
- module augmentation (9)
- generic defaults (8)
- verbatimModuleSyntax / isolatedModules (8)
- method bivariance / strictFunctionTypes (8)
- preventing distribution (7)
- compiler-performance blowups (7)
- method bivariance strictFunctionTypes (6)
- variance (co/contra) (5)
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