TypeScript Data Analysis & Reporting 4

Question 1

In a TypeScript reporting library, you want a type that represents the possible aggregate functions ('sum', 'avg', 'min', 'max', 'count'). What is the most type-safe definition?

Accepted Answer

type AggregateFunc = 'sum' | 'avg' | 'min' | 'max' | 'count'

Answer

A string literal union type restricts values to exactly the five valid aggregate functions and is exhaustively checkable with switch statements.

Question 2

What TypeScript technique allows you to create a type-safe lookup table for report formatters where each formatter's input type matches the corresponding field type in a data schema?

Accepted Answer

Use a mapped type: { [K in keyof Schema]: (val: Schema[K]) => string }

Answer

A mapped type over the schema's keys ensures each formatter receives exactly the type of the field it formats, catching type mismatches at compile time.

Question 3

When a TypeScript data processing function might return either a Report object or throw, which pattern best communicates both outcomes to callers without relying on exceptions?

Accepted Answer

Return a Result type: { ok: true; data: Report } | { ok: false; error: string }

Answer

A discriminated union Result type forces callers to handle both success and failure paths explicitly without relying on exception control flow.

Question 4

Which TypeScript feature helps ensure that a switch statement over a report metric type handles all possible cases and will cause a compile error if a new metric is added?

Accepted Answer

Exhaustiveness checking with 'never' in the default case

Answer

Assigning the unhandled value to a variable of type 'never' in the default case causes a compile error if any union member is not handled.

Question 5

A TypeScript function receives an object where values may be numbers or arrays of numbers. How do you safely compute the sum regardless of which form is used?

Accepted Answer

Check Array.isArray(value) to narrow the type before summing

Answer

Array.isArray() is a TypeScript type guard that narrows the value to number[] in the true branch and number in the false branch.

TypeScript Practice Test

TypeScript Practice Test

TypeScript Data Analysis & Reporting 4