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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

Final Check: TypeScript for Beginners

Fifteen scenario questions across the whole course. If you get twelve or more right, you have the TypeScript basics covered.

15 min TypeScriptProgramming BasicsTesting and DebuggingType Systems Beginner
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1. Why does the compiled hello.js not contain any type annotations?
The compiler removes them, because Node and browsers run JavaScript and do not understand types. Because the annotations were wrong and were dropped. Because tsc only copies the function bodies. Because annotations only work in the editor.
2. You install TypeScript with npm install --save-dev typescript. Typing tsc --version says command not found, but npx tsc --version works. Is something broken?
Yes, you must install TypeScript globally as well. No. The project install puts tsc in node_modules/.bin, which is not on your PATH, and npx looks there. Yes, the install failed and npx downloaded a copy. No, but only because npx is faster than tsc.
3. A variable is declared with let total: number; and no value. Why is the annotation needed here but not in const rate = 0.12;?
Because numbers must be annotated and decimals must not. The first has nothing to infer from, and the second gets its type from the value. Because the compiler ignores const. Because let always needs an annotation and const never does.
4. What is the safest fix when strict mode reports that null is not assignable to string for a variable you declared as string?
Change the type to any. Decide whether the variable can really be empty, and if so change its type to a union of string and null and handle that case. Turn off strictNullChecks permanently. Add as string after the null.
5. You read an array element with prices[5] on a number array of length three. In the default configuration, what is the type and what is the value at run time?
Both are number, because arrays fill missing items with zero. The compiler treats it as number, and at run time it is undefined. The compiler treats it as undefined, and at run time it is an error. The compiler refuses to compile the line.
6. Which declaration of a property means the key may be left out of the object completely?
The readonly modifier on the property. Typing the property as any. A question mark after the name, as in title?: string. A union of string and undefined, written without a question mark.
7. A function parameter is typed readonly string[]. Your code calls push on it. What happens?
It reports that string arrays cannot be changed in TypeScript. The compiler reports that push does not exist on a readonly array. It works, because readonly is only a comment. It works, but the array is copied first.
8. A function declared to return number reaches the end of one branch without a return statement. What is the benefit of the declared return type?
It makes the function run faster. It forces every function to return a number. Nothing, because the compiler infers the same thing. The compiler finds the gap when you compile, instead of the function quietly returning undefined later.
9. You have a function with three optional parameters and calls like format(100, undefined, undefined, true). What is the better design?
Make all of them required. Type them as any so the order does not matter. Add a fourth optional parameter for the order. Pass one options object with optional properties and a default of an empty object.
10. Your team keeps mixing interface and type aliases for object shapes and arguing about which one to use. What is the most useful decision?
Pick one rule for object shapes, keep type for unions and literals, and write the rule down. Use both randomly, because the compiler treats them differently. Always use interface, because type is deprecated. Always use type, because interface cannot describe objects.
11. A function takes a string or a number and uses typeof id === "string" in an if. Inside the block you call id.toUpperCase(). After the block you call id.toFixed(2). Why does the second call compile?
Because toFixed exists on strings. Because the compiler stops checking after an if block. Because you wrote as number somewhere. The string case already returned, so after the block the compiler knows id is a number.
12. You turn on strict in an old project and get hundreds of errors. What is the most practical first step?
Add @ts-ignore to every failing line. Count the errors for each strict flag, turn them on one at a time starting with the cheap ones, and fix as you go. Delete the types from the files that fail. Leave strict false and promise to come back later.
13. In a project with a tsconfig.json, which command compiles everything listed in include?
npx tsc with no file name. npx tsc hello.ts. npm install tsc. node tsc.
14. You renamed app.js to app.ts and now get Cannot find name process. What is the most likely cause and fix?
The Node type definitions are missing. Install @types/node and add node to the types field in tsconfig.json. process was removed from Node, so rewrite the program. You must rename the file back to app.js. You must declare process as any at the top of the file.
15. You see the message Parameter x implicitly has an any type. Which reading is correct?
The function is never called, so TypeScript guessed. x is a reserved name. A parameter has no type, and strict mode refuses to let it quietly be any. Write the type. The function must be async.

About

Fifteen scenario questions across the whole course. If you get twelve or more right, you have the TypeScript basics covered.

Duration
15 min
Last update
10/11/2026
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