TypeScript interview questions

Write async function safeDivide(a: number, b: number): Promise<{ ok: true; value: number } | { ok: false; error: string }> that resolves to { ok: true, value: a / b }, or, when b is 0, resolves to { ok: false, error: "Division by zero" } instead of throwing.

What does the return type of an async function always look like? Why can't an async function just return T directly?

Write a BankAccount class with a private balance starting at 0, deposit(amount: number): void, and withdraw(amount: number): boolean that subtracts amount and returns true only if the balance is sufficient (otherwise it changes nothing and returns false). Then

Write an abstract class Shape with abstract area(): number, and concrete subclasses Circle (constructed with a radius) and Rectangle (constructed with width and height). Then write totalArea(shapes: ({ kind: "circle"; radius: number } | { kind: "rectangle"; width: number; height: number })[]): number

What do public, private and protected mean on a class member? Are they enforced at runtime?

Write declarationFileNameFor(sourceFile: string): string that returns the declaration-file name for a JS source file: if it already ends with .d.ts, return it unchanged; if it ends with .js, .jsx, .mjs or .cjs, replace that extension with .d.ts; otherwise, append .d.ts