Mastering TypeScript for JavaScript Users

Which of the following is the correct file extension for a TypeScript file?

• .js

• .ts

• .jsx

• .tsx

What keyword is used in TypeScript to explicitly declare a variable's type?

• typeof

• type

• as

• :

Which of the following is NOT a valid TypeScript type?

• string

• number

• boolean

• digit

What is the default type assigned to a variable in TypeScript if no type is specified and no value is assigned?

• string

• any

• undefined

• object

Which TypeScript feature allows you to specify that a function parameter is optional?

• Using the "?" after the parameter name

• Using the "optional" keyword

• Using the "maybe" keyword

• Using the "!" after the parameter name

Given the following TypeScript code, what is the type of the variable `result`? ```typescript let result: number | string; ```

• number only

• string only

• number or string

• any

Which of the following best describes the purpose of TypeScript interfaces?

• To define the structure of objects

• To perform mathematical operations

• To create new data types at runtime

• To handle asynchronous code

How would you define a function in TypeScript that takes a parameter `x` of type `number` and returns a `string`?

• `function foo(x: string): number {}`

• `function foo(x: number): string {}`

• `function foo(x): string {}`

• `function foo(x: number): void {}`

Which of the following statements about TypeScript's "readonly" modifier is true?

• It allows a property to be changed only once.

• It prevents a property from being changed after initialization.

• It makes a property accessible only within the class.

• It allows a property to be deleted.

Given the following code, what will be the output? ```typescript let x: unknown = 5; let y: number = x; ```

• The code will compile and run successfully.

• The code will throw a runtime error.

• The code will not compile due to a type error.

• The code will assign `undefined` to `y`.