Typescript Interview Questions and Answers for 5 years experience
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What is TypeScript and why would you use it over JavaScript?
- Answer: TypeScript is a superset of JavaScript that adds optional static typing. This allows for early error detection during development, improved code maintainability and readability, especially in larger projects. It offers better tooling support, including features like autocompletion and refactoring in IDEs, which significantly boosts developer productivity. Using TypeScript over JavaScript is beneficial when working on complex applications where maintainability and scalability are paramount.
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Explain the concept of interfaces in TypeScript. Provide an example.
- Answer: Interfaces define the shape of an object, specifying the properties and their types. They enforce type checking at compile time. They don't contain implementations; they only define contracts. Example: ```typescript interface Person { firstName: string; lastName: string; age?: number; // Optional property } let user: Person = { firstName: "John", lastName: "Doe", age: 30 }; ```
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What are type aliases in TypeScript? How do they differ from interfaces?
- Answer: Type aliases create new names for existing types. They can be used for primitives, unions, intersections, tuples, and other types. Interfaces, on the other hand, primarily define the shape of an object. Key differences include that type aliases can be used to alias any type, while interfaces are mostly used to define object shapes. Interfaces can be extended, while type aliases cannot. Multiple interfaces can be merged, but multiple type aliases cannot.
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Explain the concept of generics in TypeScript. Give a practical example.
- Answer: Generics allow you to write reusable code components that can work with different types without losing type safety. They use type parameters to represent unknown types, which are then inferred or explicitly specified when the component is used.
Example:
```typescript
function identity
(arg: T): T { return arg; } let myString: string = identity ("hello"); let myNumber: number = identity (10); ```
- Answer: Generics allow you to write reusable code components that can work with different types without losing type safety. They use type parameters to represent unknown types, which are then inferred or explicitly specified when the component is used.
Example:
```typescript
function identity
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What are tuples in TypeScript? Provide an example.
- Answer: Tuples are arrays with a fixed number of elements, where each element has a specific type. Example: ```typescript let myTuple: [string, number, boolean] = ["hello", 10, true]; ```
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Explain union and intersection types in TypeScript.
- Answer: Union types allow a variable to be one of several types. Intersection types combine multiple types into one, requiring an object to have all properties of all constituent types. Example: ```typescript // Union type let value: string | number = 10; value = "hello"; // Intersection type interface Person { name: string; } interface Worker { job: string; } let personWorker: Person & Worker = { name: "John", job: "Developer" }; ```
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What is type assertion in TypeScript? When would you use it?
- Answer: Type assertion tells the TypeScript compiler to treat a variable as a different type than its inferred type. It's used when you know more about the type than the compiler does, but use it cautiously as it can bypass type safety. It's done using angle-bracket syntax or `as` syntax.
Example:
```typescript
let value: any = "hello";
let strLength: number = (
value).length; // Angle-bracket syntax let strLength2: number = (value as string).length; // 'as' syntax ```
- Answer: Type assertion tells the TypeScript compiler to treat a variable as a different type than its inferred type. It's used when you know more about the type than the compiler does, but use it cautiously as it can bypass type safety. It's done using angle-bracket syntax or `as` syntax.
Example:
```typescript
let value: any = "hello";
let strLength: number = (
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Explain the difference between `let`, `const`, and `var` in TypeScript.
- Answer: `let` declares a block-scoped variable, mutable after declaration. `const` declares a block-scoped variable, immutable after initialization. `var` declares a function-scoped variable, mutable after declaration. `let` and `const` are preferred for their better scoping and preventing accidental modification.
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How do you handle optional properties in TypeScript interfaces and classes?
- Answer: Optional properties are declared using a question mark `?` after the property name in an interface or class definition. This indicates that the property may or may not be present in an object that conforms to the interface or is an instance of the class.
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What are namespaces in TypeScript? When would you use them?
- Answer: Namespaces provide a way to organize code into logical units, preventing naming conflicts. They are less common now with the advent of modules (using `import` and `export`), but can still be useful in some scenarios, particularly when working with older codebases or libraries. Modules are generally preferred for modern TypeScript development.
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Explain the role of `this` keyword in TypeScript classes and functions.
- Answer: The `this` keyword refers to the current instance of a class or object. In methods within a class, `this` refers to the instance of that class. In functions, `this` depends on how the function is called (e.g., explicitly bound or within a class method). Arrow functions lexically bind `this`, which means `this` refers to the surrounding scope.
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How do you implement inheritance in TypeScript? Provide an example.
- Answer: Inheritance is implemented using the `extends` keyword. A class can extend another class, inheriting its properties and methods. Example: ```typescript class Animal { name: string; constructor(name: string) { this.name = name; } makeSound() { console.log("Generic animal sound"); } } class Dog extends Animal { breed: string; constructor(name: string, breed: string) { super(name); // Call superclass constructor this.breed = breed; } makeSound() { console.log("Woof!"); } } ```
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What are access modifiers in TypeScript (public, private, protected)?
- Answer: Access modifiers control the accessibility of class members. `public` members are accessible from anywhere. `private` members are only accessible within the class they are declared in. `protected` members are accessible within the class and its subclasses.
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How do you work with modules in TypeScript?
- Answer: Modules in TypeScript are used to organize code into separate files. They use `export` to make declarations available to other modules and `import` to bring in declarations from other modules. This promotes code reusability and maintainability.
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Explain the concept of decorators in TypeScript.
- Answer: Decorators are a way to add metadata or modify classes, methods, or properties using a declarative syntax. They are functions that take a target (class, method, etc.) and add functionality to it.
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How do you handle asynchronous operations in TypeScript using promises and async/await?
- Answer: Promises represent the eventual result of an asynchronous operation. `async/await` provides a cleaner syntax for working with promises, making asynchronous code look and behave more like synchronous code.
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What are mapped types in TypeScript? Provide an example.
- Answer: Mapped types allow you to create new types based on existing types. They iterate over the properties of an existing type and transform them according to a specified mapping.
Example:
```typescript
interface Person {
name: string;
age: number;
}
type ReadonlyPerson = Readonly
; //Creates a readonly version of Person ```
- Answer: Mapped types allow you to create new types based on existing types. They iterate over the properties of an existing type and transform them according to a specified mapping.
Example:
```typescript
interface Person {
name: string;
age: number;
}
type ReadonlyPerson = Readonly
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Explain conditional types in TypeScript.
- Answer: Conditional types allow you to define types that depend on a condition. They use a ternary-like syntax to select one type or another based on whether a condition is true or false.
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What are some common TypeScript design patterns you've used?
- Answer: (This answer should list several design patterns relevant to TypeScript and the candidate's experience, such as Singleton, Factory, Observer, Decorator, Strategy, etc., with brief explanations of their application in TypeScript.)
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How do you handle type errors in TypeScript?
- Answer: TypeScript's compiler will identify type errors during compilation. Addressing these involves examining the error messages, understanding why the types are mismatched, and correcting the code to match the expected types. Using tools like type assertions cautiously (when absolutely necessary) might be part of the solution, but focusing on correctly defining types and ensuring data flows correctly is key.
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How do you test your TypeScript code?
- Answer: (This answer should discuss various testing methodologies used, e.g., unit testing with Jest, Mocha, or similar frameworks, integration testing, and potentially end-to-end testing, explaining the approach and the tools used.)
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What are some common challenges you've faced while working with TypeScript, and how did you overcome them?
- Answer: (The answer here should reflect the candidate's experience. Examples might include managing large type definitions, dealing with legacy codebases that lack types, handling complex generics, or integrating TypeScript with other technologies. The key is to show problem-solving skills and a willingness to learn.)
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Explain your experience with TypeScript in a large-scale project.
- Answer: (This answer should describe a relevant project, highlighting challenges, solutions, and the impact of TypeScript on the project's success. The candidate should focus on how TypeScript improved code quality, maintainability, and scalability.)
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How do you stay up-to-date with the latest changes and features in TypeScript?
- Answer: (The candidate should mention resources used to stay current, such as the official TypeScript website, blogs, articles, conferences, and the TypeScript community.)
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Describe your experience using TypeScript with different frameworks or libraries (e.g., React, Angular, Node.js).
- Answer: (This should detail specific projects and the frameworks used, highlighting the integration process and any challenges faced. It should demonstrate familiarity with the tooling and best practices involved.)
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What are some best practices you follow when writing TypeScript code?
- Answer: (This answer should cover a range of best practices, including consistent naming conventions, proper use of interfaces and types, effective use of generics, modularity, and testing strategies.)
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How would you approach refactoring a large TypeScript codebase that lacks type definitions?
- Answer: (This should outline a strategic approach, including prioritizing areas for refactoring, using gradual type additions, automated tooling, testing strategies, and collaboration techniques.)
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Explain your experience with TypeScript's declaration files (.d.ts).
- Answer: (This should showcase understanding of how declaration files provide type information for JavaScript libraries, how to create them, and how to use them effectively.)
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What are some tools and techniques you use for debugging TypeScript code?
- Answer: (The candidate should mention debuggers integrated into IDEs, logging techniques, and how to interpret compiler error messages effectively.)
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How do you handle null and undefined values in TypeScript to avoid runtime errors?
- Answer: (This should cover the use of optional chaining (`?.`), nullish coalescing (`??`), and the non-nullable type (`!`) explaining their usage and trade-offs. The use of types like `string | null` or `number | undefined` to explicitly handle potential null or undefined should also be mentioned.)
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Describe your experience with different TypeScript compiler options and their impact on code compilation and performance.
- Answer: (This response requires the candidate to demonstrate familiarity with common compiler options like `target`, `module`, `strict`, `esModuleInterop`, and the implications of different settings on code compatibility, output size, and compilation speed.)
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