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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: Functions That State Their Deal

Five scenario questions on parameter types, return types and defaults. If you miss more than one, go back to the two lessons before moving on.

5 min TypeScriptProgramming BasicsTesting and DebuggingType Systems Beginner
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1. You write function add(a, b) { return a + b; } in a strict project and get Parameter a implicitly has an any type. What is the right fix?
Add a return type only. Change the parameters to any. Turn strict off in tsconfig.json. Give each parameter a real type, for example a: number and b: number.
2. A function declared as returning string has an if branch that returns "pass" and nothing after the if. What does the compiler report?
Nothing, because JavaScript returns undefined and that is fine. That the function should return a number. That the function lacks an ending return statement, because some path returns nothing. That the if statement is not allowed.
3. A function does rate = rate || 0.12; and a customer has a rate of 0. The tax is calculated as if the rate were 0.12. What is the cleaner fix?
Make rate required and ignore customers with no rate. Use a default parameter such as rate = 0.12, which only applies when the argument is undefined. Use rate && 0.12 instead. Cast rate with as number.
4. You write function format(currency?: string, price: number). What does the compiler say and why?
Nothing, optional parameters can be anywhere. That currency must be a number. That price should also be optional. A required parameter cannot follow an optional one, because a caller cannot skip a parameter in the middle.
5. You call prices.map((p) => p.toUpperCase()) where prices is a number array. You never annotated p. Why does the compiler complain?
Because toUpperCase must be called with an argument. The type of p is inferred from the array, so it is a number and numbers have no toUpperCase. Because map only works on arrays of strings. Because p has no annotation, so it is an error to leave it off.

About

Five scenario questions on parameter types, return types and defaults. If you miss more than one, go back to the two lessons before moving on.

Duration
5 min
Last update
10/16/2026
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