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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: Before You Write Types

Five scenario questions on what the compiler checks and what it leaves alone. If you miss more than one, reread the first two lessons.

5 min TypeScriptProgramming BasicsTesting and DebuggingType Systems Beginner
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1. A teammate says TypeScript makes the app safer because it checks every value against its type while the app is running. What is the accurate correction?
TypeScript only checks code in the browser, not in Node. TypeScript makes the code faster because the types help the engine. The types are removed when the code is compiled, so the compiler checks your code before it runs and nothing is checked afterwards. That is right, TypeScript adds run-time checks to the compiled file.
2. In JavaScript, add("5", 3) returns "53" and nothing complains. You declare add with two number parameters in TypeScript. What happens with the same call?
The call compiles and returns 8, because TypeScript converts the string. The call compiles and still returns "53". The compiler reports an error on that call, because a string is not assignable to a number parameter. The program crashes with a TypeError when it runs.
3. You write JSON.parse(text) as { name: string; age: number } and the compiler is happy. The JSON has no age, and age.toFixed() later fails with a TypeError. What does this show?
TypeScript has a bug in how it reads JSON. Casting with as is the recommended way to validate JSON. The error should have been caught, so you need a newer version of the compiler. A type is a claim you make to the compiler. It does not check data that arrives while the program runs.
4. You run npx tsc hello.ts. It prints an error, and you see that hello.js was written anyway. Why?
By default tsc reports the errors and still writes the JavaScript file. tsc wrote hello.js from an earlier run and did not update it. The error was not serious enough to count. tsc cannot find errors in .ts files until it has written the .js file.
5. You run npx tsc hello.ts and it prints nothing at all. What does that mean?
The file was skipped because it has no types. There were no errors, and hello.js was written next to hello.ts. The compiler is waiting for you to press a key. Nothing happened, so the install is broken.

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Five scenario questions on what the compiler checks and what it leaves alone. If you miss more than one, reread the first two lessons.

Duration
5 min
Last update
10/10/2026
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