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TypeScript Intermediate: Types That Survive Real Projects
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TypeScript Intermediate: Types That Survive Real Projects

Check Yourself: Working With Code You Didn't Write

Five scenario questions on declaration files, types packages and typing fetch results. If you miss more than one, reread the lessons in this section.

5 min TypeScriptObject-Oriented ProgrammingAPIs and IntegrationsType Systems Intermediate
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1. You install lodash, import it in a TypeScript file in strict mode, and tsc reports TS7016, could not find a declaration file for the module. What does the message mean?
Strict mode does not support libraries written in JavaScript. lodash is not installed, so run npm install again. The import is wrong, and it should use require instead. The compiler found the JavaScript but nothing that describes its types. Install the types package, or add a declaration file as a stopgap.
2. You run npm view on a library with the field name types, and it prints nothing. What is the next step?
The library is broken and should not be used. Add the library to dependencies a second time. The library ships no types of its own, so look for a types package from DefinitelyTyped and install it as a development dependency. Set the types option in tsconfig to the library name.
3. A declaration file says findRate returns number, but the real code returns undefined for a currency it does not know. A line calls findRate for EUR and then calls toFixed on the result. What is the safest fix?
Report nothing, because the compiler accepted it. Wrap the call in your own function that declares number or undefined as the return type, so callers have to check. Add an as any cast at each call. Declare the variable as number or undefined at the call and assign the result.
4. A function calls fetch, then returns the parsed body with an as Todo cast. The server answers 404 with an empty object. What does the caller get?
A compile-time error, because the status is 404. A Todo with default values filled in. A run-time error from the cast itself. An object that the compiler believes is a Todo, with title undefined at run time. The cast checked nothing and ignored the status.
5. You write function isTodo with a parameter of type unknown and a return type of value is Todo. What does that return type tell the compiler?
The value is a Todo even when the function returns false. When the function returns true, the argument can be treated as a Todo from then on. The function always returns the value itself. The function converts the value into a Todo.

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Five scenario questions on declaration files, types packages and typing fetch results. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/14/2026
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