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.

Write async function runPipeline(steps: number[], failOn: number): Promise<{ results: number[]; error: string | null }>. Process steps one at a time, in order, awaiting an async step that doubles the value — but if a step's value equals failOn, that step throws a custom PipelineError (carrying the step's index and a

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

In a try/catch around an await, what type does the caught error have in modern TypeScript, and how do you use it safely?

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