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TypeScript for Beginners: Catch Wrong Data Before It Runs
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TypeScript for Beginners: Catch Wrong Data Before It Runs

Check Yourself: Shapes You Can Reuse

Five scenario questions on type aliases, merging, literal widening and narrowing. If you miss more than one, reread the lessons in this section.

5 min TypeScriptProgramming BasicsTesting and DebuggingType Systems Beginner
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1. You need a type that is exactly one of the words draft, sent or paid. Which tool can name it?
An interface with three properties. A type alias that joins the three words into a union. A readonly array of strings. The any type with a comment.
2. Two files in the same global scope each declare interface Settings with different properties. What happens, and what did you probably not expect?
The second declaration silently replaces the first. The compiler reports a duplicate name error on the second one. The two declarations merge into one Settings with all the properties, so an object with only some of them is missing a property. Nothing, because they are in different files.
3. You declare const order = { status: "paid" }; and call label(order.status) where label accepts a union of the words draft, sent and paid. The call fails. Why?
The status property was widened to string, because object properties can change later. Because label must be declared as async. Because const objects cannot be passed to functions. paid is not one of the allowed words.
4. A union has two object types that both have a name property and differ in other keys. You write if ("name" in person) and then read person.id. Why does the compiler refuse?
Because id is a reserved word. Both members have name, so the check does not narrow. Check a key that exists in only one member, such as id. Because person must first be cast to any. The in operator does not work with objects.
5. A parameter is typed object or null. You write if (typeof value === "object") and call value.toString() inside. The compiler says value is possibly null. Why?
Because typeof only works on strings and numbers. Because value should have been typed any. In JavaScript typeof null is object, so the check does not rule out null. Compare with value !== null. Because toString does not exist on objects.

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Five scenario questions on type aliases, merging, literal widening and narrowing. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/10/2026
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