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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: Types You Cannot Skip

Five scenario questions on inference, null, shapes, optional properties and any. 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 write let count = 0; and a few lines later count = "zero";. There is no annotation anywhere. What does the compiler do?
It accepts the line only if you add the as keyword. It reports an error, because it inferred number from the first value. It accepts the line and changes count to a string from then on. It accepts the line, because count was never given a type.
2. A new file contains only let name: string = "Ana"; and tsc reports Cannot redeclare block-scoped variable name. You declared it once. What is going on and what is the fix?
The compiler counts the string Ana as a second declaration. The word name is reserved by JavaScript and can never be a variable. You need to write var instead of let. A file with no import or export is a script that shares the global scope with the browser definitions, which already declare name. Add export {} or pick another name.
3. You write const product: { title: string; price: number } = { title: "Pen" };. What does the compiler tell you?
That title should be a number. That the object has an extra property. Nothing, because a missing property just becomes undefined. The price property is missing from the object but required by the type.
4. A type has subtitle?: string. Your code calls product.subtitle.toUpperCase() directly. The compiler refuses. Why?
The subtitle may be undefined, so you have to check it, give it a default or use optional chaining first. toUpperCase is not a string method. Optional properties are always strings, so this is a compiler bug. Optional properties cannot be read at all.
5. A function copies a value out of a variable typed any into a variable typed number. The value is really the string "three", and a later call to toFixed fails. Where did the checking stop?
Nowhere, the compiler should have caught it. At toFixed, which does not exist on strings. At the any. Everything taken from an any value is accepted without a check, so the wrong type spread silently. At the number annotation, which is ignored when an any is involved.

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Five scenario questions on inference, null, shapes, optional properties and any. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/12/2026
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