TypeScript Variables and Data Types Fundamentals Quiz Quiz

Challenge your understanding of TypeScript variables, type declarations, and the differences between fundamental data types. This interactive quiz covers important concepts such as variable initialization, assignment, and common typing pitfalls in TypeScript, designed for effective learning and review.

1. Variable Declaration Syntax

   Which of the following is the correct way to declare a typed variable holding a number value in TypeScript?

   1. let age number = 25;
   2. var age = number 25;
   3. let age: number = 25;
   4. const age: num = 25;

   Explanation: The correct syntax for declaring a typed variable in TypeScript is 'let age: number = 25;'. Option B uses incorrect syntax by placing the keyword 'number' in the wrong place. Option C uses 'num' instead of the correct 'number' type. Option D leaves out the colon, which is required for type annotation.

2. Type Inference Behavior

   What type will TypeScript infer for the variable declared as 'let isActive = false;'?

   1. undefined
   2. any
   3. boolean
   4. string

   Explanation: TypeScript infers the variable 'isActive' as a boolean since it is initialized with the boolean value 'false'. Option B, 'any', only occurs without an assignment or an explicit type. Option C, 'string', would require a string value, and Option D, 'undefined', applies if no value is assigned.

3. Difference Between 'any' and 'unknown'

   If a variable is declared with the 'unknown' type, what must a developer do before assigning it to a variable with a specific type?

   1. Convert it using JSON.stringify()
   2. Assign it directly without checks
   3. Perform a type assertion or type checking first
   4. Use 'unknown' only with arrays

   Explanation: When using the 'unknown' type, developers are required to perform a type assertion or runtime type check before assigning its value to another typed variable. Assigning directly without a check (option B) is disallowed for safety reasons. Option C, converting with JSON.stringify(), is irrelevant and does not change the variable type. Option D is incorrect, as 'unknown' can be used with any type, not just arrays.

4. Const vs. Let

   What happens if you try to reassign a variable defined with 'const' in TypeScript?

   1. The value changes only in strict mode
   2. The variable silently updates
   3. A compile-time error occurs
   4. It becomes undefined

   Explanation: Reassigning a variable declared with 'const' results in a compile-time error, as 'const' marks variables as immutable. Option B is incorrect since TypeScript enforces immutability at compile time. Option C, becoming undefined, is not the case for reassigned constants. Option D's statement about strict mode is incorrect; immutability applies regardless of mode.

5. Assigning Null to Variables

   Given 'let username: string;', what happens if you assign null to 'username' without enabling strictNullChecks?

   1. It is allowed because 'null' is assignable to 'string'
   2. The assignment is ignored
   3. Only 'undefined' can be assigned
   4. It causes a compile-time error

   Explanation: Without 'strictNullChecks' enabled, TypeScript allows 'null' to be assigned to variables of type 'string', treating 'null' and 'undefined' as valid values. Option B is incorrect because the error only occurs when 'strictNullChecks' is enabled. Option C is wrong since the assignment does take effect. Option D is incorrect, as both 'null' and 'undefined' can be assigned without strict null checks.