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TypeScript Advanced: Types That Hold Up in Production
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TypeScript Advanced: Types That Hold Up in Production

Check Yourself: Brands and Boundaries

Five scenario questions on branded types, satisfies and as const. If you miss more than one, reread the lessons in this section.

5 min TypeScriptTesting and DebuggingPerformance OptimizationType SystemsPackaging and Dependencies Advanced
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1. Why does a function with two string parameters, userId and orderId, accept the arguments in the wrong order?
Because userId and orderId are reserved names. TypeScript compares shapes, and both parameters are plain strings, so an alias such as type UserId = string adds no new type. Because function parameters are never checked in strict mode. Because the compiler cannot compare string types.
2. What is a branded type, and what exists at run time?
A type that checks the value each time it is used. A class with a hidden field. At run time the value is an object with that field. A string with a special prefix added by the compiler. A primitive intersected with an object type that carries a tag. At run time the value is still a plain string.
3. Where should the cast from string to UserId live?
In the type declaration of UserId itself. Nowhere, because a brand needs no cast. At every call site, so each caller states what it means. In one parse function, right after the check that justifies it, so reviewers have one place to look.
4. A price in centavos is a branded number. You compute price times 2. What is the type of the result?
A plain number, so you brand it again in a small function that names the unit. The same branded type, because the brand is part of the value. A string, because a brand is always a string. never, because arithmetic on a brand is not allowed.
5. You declare const config = { port: 8080, host: "localhost" } satisfies Config. How does this differ from a plain annotation, const config: Config?
There is no difference. The two are different spellings of the same thing. satisfies converts the value into a Config at run time. satisfies checks the value against Config and keeps the precise type, so config.port is still a number. satisfies skips the check and only changes the name of the type.

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Five scenario questions on branded types, satisfies and as const. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/11/2026
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