Typescript Interview Questions

Checkout Vskills Interview questions with answers in Typescript to prepare for your next job role. The questions are submitted by professionals to help you to prepare for the Interview.

Q.1 What is TypeScript and how does it differ from JavaScript?

TypeScript is a statically-typed superset of JavaScript that adds optional static typing to the language. It compiles down to plain JavaScript and can be used in any JavaScript project. The main difference is that TypeScript introduces static types, which allow for better code organization, improved tooling support, and enhanced code scalability.

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Q.2 How do you define a variable with a specific type in TypeScript?

In TypeScript, you can define a variable with a specific type using the colon (:) syntax.

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Q.3 What are the benefits of using TypeScript?

TypeScript provides several benefits, including: Static typing catches errors at compile-time, leading to more reliable code. Improved code maintainability and scalability. Enhanced tooling support, such as autocompletion and refactoring tools. ES6+ features can be used even if the target environment doesn't support them. Better code organization through the use of interfaces, classes, and modules.

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Q.4 How can you define a function with optional parameters in TypeScript?

To define a function with optional parameters in TypeScript, you can use the question mark (?) syntax.

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Q.5 What is the difference between "undefined" and "null" in TypeScript?

In TypeScript, both "undefined" and "null" represent absence of a value, but they are used in slightly different contexts. "undefined" is typically used when a variable has been declared but has not been assigned a value, while "null" is usually assigned to indicate that a variable intentionally holds no value.

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Q.6 How can you declare and use an interface in TypeScript?

To declare an interface in TypeScript, you can use the interface keyword.

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Q.7 What is the purpose of the "readonly" modifier in TypeScript?

The "readonly" modifier is used to indicate that a property of an object can only be read and cannot be modified. It ensures that the property remains constant after initialization.

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Q.8 How can you achieve inheritance in TypeScript?

In TypeScript, you can achieve inheritance by using the extends keyword.

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Q.9 How can you handle asynchronous operations in TypeScript?

TypeScript provides several ways to handle asynchronous operations, such as Promises, async/await syntax, and Observables (when using libraries like RxJS). Promises are a common approach that allows you to work with asynchronous data and perform operations once the data is available.

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Q.10 How can you compile TypeScript code to JavaScript?

To compile TypeScript code to JavaScript, you can use the TypeScript compiler (tsc) either through the command line or through build tools like Webpack or Babel. The compiler takes TypeScript files (with the .ts extension) and generates equivalent JavaScript files (with the .js extension). The resulting JavaScript code can then be executed in any JavaScript runtime environment.

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Q.11 What are the basic types in TypeScript?

The basic types in TypeScript include: number: Represents numeric values, including integers and floating-point numbers. string: Represents textual data. boolean: Represents a logical value (true or false). null: Represents the absence of a value. undefined: Represents a value that has not been assigned. object: Represents a non-primitive type, such as arrays, functions, or objects. array: Represents an ordered list of values of a specific type. tuple: Represents an array with a fixed number of elements of different types. enum: Represents a set of named constants. any: Represents any type, allowing dynamic typing.

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Q.12 How can you explicitly specify the type of a variable in TypeScript?

In TypeScript, you can explicitly specify the type of a variable using the colon (:) syntax. For example: let age: number = 25; Here, the variable age is explicitly defined as a number type.

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Q.13 What is the difference between "let" and "const" in TypeScript?

In TypeScript, let and const are used for variable declarations. The main difference is that variables declared with let can be reassigned to a new value, while variables declared with const are read-only and cannot be reassigned once they are initialized.

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Q.14 How can you define an array with a specific type in TypeScript?

To define an array with a specific type in TypeScript, you can use the type followed by square brackets ([]). For example: let numbers: number[] = [1, 2, 3, 4, 5]; Here, the variable numbers is defined as an array of numbers.

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Q.15 What is the "union" type in TypeScript?

The "union" type in TypeScript allows a variable to hold values of different types. It is represented using the pipe symbol (|) between the types. For example: let result: string | number; result = "Success"; result = 200; Here, the variable result can hold either a string or a number.

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Q.16 How can you define a function with a specific return type in TypeScript?

To define a function with a specific return type in TypeScript, you can use the colon (:) syntax after the parameter list.

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Q.17 What is the "any" type in TypeScript?

The "any" type in TypeScript represents a dynamic type that allows for flexible typing. Variables of type "any" can hold values of any type, and TypeScript compiler doesn't perform type-checking on them. It is often used when working with existing JavaScript code or when the type of a value is unknown or can vary.

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Q.18 What is a tuple in TypeScript?

A tuple in TypeScript is an array-like structure that allows storing a fixed number of elements of different types. The types and the order of the elements are defined at the time of declaration.

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Q.19 What is the "never" type in TypeScript?

The "never" type in TypeScript represents a type for values that will never occur. It is typically used to indicate that a function will never return or that a variable cannot have any value.

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Q.20 How can you perform type assertions in TypeScript?

Type assertions in TypeScript allow you to explicitly specify the type of a value when you know more about the value than TypeScript can infer. You can use the angle bracket syntax () or the as keyword.

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Q.21 What are control flow statements in TypeScript?

Control flow statements in TypeScript are used to control the execution flow of a program. They include conditional statements (if-else, switch), loop statements (for, while, do-while), and branching statements (break, continue, return).

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Q.22 How does the "if-else" statement work in TypeScript?

The "if-else" statement in TypeScript allows you to conditionally execute a block of code. If the specified condition evaluates to true, the code within the "if" block is executed. Otherwise, if the condition evaluates to false, the code within the "else" block (if provided) is executed.

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Q.23 What is the purpose of the "switch" statement in TypeScript?

The "switch" statement in TypeScript allows you to perform different actions based on different conditions. It evaluates an expression and compares it against multiple case values. If a case value matches the expression, the corresponding code block is executed. The "switch" statement also supports a default case that is executed if no match is found.

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Q.24 How does the "for" loop work in TypeScript?

The "for" loop in TypeScript allows you to iterate over a block of code for a specific number of times. It consists of an initialization statement, a condition, an increment/decrement expression, and the code block to be executed.

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Q.25 What is the difference between "while" and "do-while" loops in TypeScript?

The "while" and "do-while" loops in TypeScript are used to repeat a block of code as long as a certain condition is true. The main difference is that the "while" loop checks the condition at the beginning, and if it evaluates to false, the code is not executed at all. On the other hand, the "do-while" loop executes the code block at least once and then checks the condition at the end.

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Q.26 How does the "break" statement work in TypeScript?

The "break" statement in TypeScript is used to exit or terminate a loop or switch statement prematurely. When encountered, the control flow immediately exits the enclosing loop or switch statement. It is often used when a certain condition is met, and you want to stop the execution of the loop or switch block.

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Q.27 How does the "continue" statement work in TypeScript?

The "continue" statement in TypeScript is used to skip the rest of the code within a loop iteration and move to the next iteration. When encountered, the control flow jumps to the next iteration of the loop, bypassing the remaining code in the current iteration. It is often used when you want to skip certain iterations based on a specific condition.

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Q.28 How does the "return" statement work in TypeScript?

The "return" statement in TypeScript is used to exit a function and optionally return a value back to the caller. When encountered, the control flow immediately exits the function, and if a value is specified after the "return" keyword, it is returned to the caller. It is often used to terminate the execution of a function prematurely or provide a result back to the caller.

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Q.29 What is an "infinite loop," and how can you break out of it?

An "infinite loop" is a loop that continuously executes without ever terminating. It occurs when the condition for exiting the loop is never met or when there is no condition specified. To break out of an infinite loop, you can use techniques such as adding a break statement inside the loop when a specific condition is met, using a control variable to control the loop termination, or using the "return" statement to exit the function enclosing the loop.

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Q.30 How can you use control flow statements effectively to handle errors in TypeScript?

Control flow statements can be used effectively to handle errors in TypeScript by combining them with error handling mechanisms such as try-catch blocks. By using try-catch blocks, you can execute a block of code that might throw an error and handle that error gracefully in the catch block. This allows you to control the flow of execution and handle different scenarios based on the occurrence of errors.

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Q.31 What is a function in TypeScript?

A function in TypeScript is a block of code that performs a specific task or calculates a value. It can take parameters as inputs, perform operations, and return a value. Functions are reusable and help in organizing and modularizing code.

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Q.32 How do you declare a function in TypeScript?

To declare a function in TypeScript, you can use the function keyword followed by the function name, a parameter list within parentheses, and an optional return type.

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Q.33 What is the difference between parameters and arguments in TypeScript?

In TypeScript, parameters are the variables listed in the function declaration, representing the inputs that the function expects. Arguments, on the other hand, are the actual values passed to the function when it is called. Parameters and arguments are related: when you call a function, you provide arguments that match the function's parameters.

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Q.34 How can you specify optional parameters in a function in TypeScript?

To specify optional parameters in a function, you can add a question mark (?) after the parameter name in the function declaration. Optional parameters can have default values assigned to them.

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Q.35 What is the rest parameter in TypeScript?

The rest parameter in TypeScript allows a function to accept any number of arguments as an array. It is denoted by using three dots (...) before the parameter name in the function declaration. The rest parameter must be the last parameter in the function signature.

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Q.36 How can you define the return type of a function in TypeScript?

To define the return type of a function in TypeScript, you can use the colon (:) syntax followed by the desired return type after the parameter list.

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Q.37 What are arrow functions in TypeScript?

Arrow functions, also known as fat arrow functions, provide a concise syntax for writing functions in TypeScript. They are a shorthand notation for writing anonymous functions. Arrow functions have a lexically bound this keyword, and they don't have their own this context.

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Q.38 What is a higher-order function in TypeScript?

A higher-order function in TypeScript is a function that takes one or more functions as arguments or returns a function as its result. It allows for functional composition and abstraction of common patterns. Higher-order functions are a powerful concept in functional programming.

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Q.39 How can you define function overloads in TypeScript?

Function overloads in TypeScript allow you to provide multiple function signatures for a single function. They enable the function to accept different combinations of argument types and return types. Overloads are declared using the function keyword followed by the function name and different signatures using the declare keyword.

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Q.40 How can you use the "this" keyword in TypeScript functions?

The this keyword in TypeScript functions refers to the object that the function is bound to at runtime. It allows you to access properties and methods of the object within the function. The behavior of this depends on how the function is called or invoked. It can be explicitly bound using the bind method, or it can be automatically bound in certain contexts, such as object methods or arrow functions. Proper understanding and usage of the this keyword are essential for working with object-oriented programming and class-based systems.

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Q.41 What is a class in TypeScript?

A class in TypeScript is a blueprint for creating objects that encapsulates data and behavior. It serves as a template or a structure that defines the properties and methods an object of that class will have.

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Q.42 How do you declare a class in TypeScript?

To declare a class in TypeScript, you can use the class keyword followed by the class name. Within the class body, you can define properties, methods, and constructors.

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Q.43 What is the purpose of a constructor in a class?

A constructor in a class is a special method that is automatically invoked when an object of that class is created. It is used to initialize the properties of the object with the provided values. Constructors allow you to set up the initial state of an object and perform any necessary setup operations.

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Q.44 How can you create an object from a class in TypeScript?

To create an object from a class in TypeScript, you can use the new keyword followed by the class name and parentheses. The parentheses can contain any arguments required by the class's constructor.

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Q.45 What is the difference between properties and methods in a class?

Properties in a class represent the data or state associated with an object. They hold values and define the characteristics of an object. Methods, on the other hand, represent the behavior or actions that an object can perform. They are functions defined within a class and operate on the object's data. Properties store values, while methods perform operations or computations.

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Q.46 How do you define and use inheritance in TypeScript classes?

In TypeScript, you can define and use inheritance by using the extends keyword. It allows a class (subclass) to inherit properties and methods from another class (superclass or base class). The subclass can add additional properties and methods or override the inherited ones.

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Q.47 What are access modifiers in TypeScript classes?

Access modifiers in TypeScript classes control the visibility and accessibility of properties and methods within a class and its subclasses. TypeScript provides three access modifiers: public: The default access modifier. Public properties and methods are accessible from anywhere. private: Private properties and methods are only accessible within the class that defines them. protected: Protected properties and methods are accessible within the class that defines them and their subclasses. By specifying the appropriate access modifier, you can control the encapsulation and accessibility of class members.

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Q.48 What is the difference between the "static" and "instance" members of a class?

Static members in a class are associated with the class itself, rather than with specific instances of the class. They are shared among all instances of the class and can be accessed without creating an object. Instance members, on the other hand, are associated with individual instances of the class. They can only be accessed through an object of the class. Static members are defined using the static keyword, while instance members are defined without it.

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Q.49 How do you implement interfaces in TypeScript classes?

To implement interfaces in TypeScript classes, you can use the implements keyword followed by the interface name. The class must provide implementations for all the members defined in the interface. This ensures that the class adheres to the contract specified by the interface.

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Q.50 How can you achieve method overloading in TypeScript classes?

Method overloading in TypeScript classes allows you to provide multiple method signatures for a single method name. It enables the method to accept different combinations of arguments or have different return types. Overloaded methods are declared using the declare keyword and different signatures. The actual implementation of the method is provided in the class.

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Q.51 What is an interface in TypeScript?

An interface in TypeScript is a way to define the structure of an object. It specifies the properties and methods that an object should have, without providing any implementation details. Interfaces allow for the creation of contracts that enforce consistency and provide a common understanding of the expected shape of objects.

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Q.52 How do you declare an interface in TypeScript?

To declare an interface in TypeScript, you can use the interface keyword followed by the interface name. Inside the interface body, you define the properties and methods that should be present in objects implementing the interface.

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Q.53 Can a class implement multiple interfaces in TypeScript?

Yes, a class in TypeScript can implement multiple interfaces. You can use the implements keyword followed by a comma-separated list of interface names. The class must provide implementations for all the members defined in the interfaces it implements.

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Q.54 What is the purpose of optional properties in interfaces?

Optional properties in interfaces allow for flexibility in object structures. They are denoted by adding a question mark (?) after the property name. Optional properties can be present or omitted in objects that implement the interface. This is useful when certain properties may not be required in all instances.

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Q.55 Can interfaces have readonly properties in TypeScript?

Yes, interfaces in TypeScript can have readonly properties. Readonly properties can be assigned a value only during object initialization and cannot be modified thereafter. They are denoted by using the readonly modifier before the property name.

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Q.56 How can you extend an interface from another interface in TypeScript?

To extend an interface from another interface in TypeScript, you can use the extends keyword. This allows one interface to inherit the properties and methods of another interface. The extending interface must provide implementations for all the inherited members.

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Q.57 What is the difference between interfaces and classes in TypeScript?

Interfaces and classes in TypeScript serve different purposes. Interfaces define the structure and contract that an object should adhere to, specifying properties and methods without providing implementations. Classes, on the other hand, are blueprints for creating objects, providing both structure and implementation details. Classes can implement interfaces to fulfill the contract defined by the interface.

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Q.58 Can you create an object directly from an interface in TypeScript?

No, you cannot create an object directly from an interface in TypeScript. Interfaces are purely for defining the structure and contract of an object and do not provide implementations. Objects need to be created from classes that implement the interface.

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Q.59 What is the benefit of using interfaces in TypeScript?

Interfaces provide several benefits in TypeScript: They define clear contracts that objects must adhere to, ensuring consistency and avoiding unexpected behavior. They enable code reusability by allowing objects of different classes to be treated uniformly when they implement the same interface. They facilitate code organization and modularity by providing a way to describe the shape and capabilities of objects. They improve code maintainability and readability by providing documentation and structure for objects and their interactions.

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Q.60 Can interfaces be used to describe function types in TypeScript?

Yes, interfaces can be used to describe function types in TypeScript. This is known as function typing. By defining an interface with a call signature, you can specify the parameter types and return type of a function. Functions that match the defined signature can then be considered as implementing that interface.

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Q.61 What are advanced types in TypeScript?

Advanced types in TypeScript refer to the more specialized and powerful type features provided by the language. They go beyond the basic types and allow for more precise and expressive type definitions.

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Q.62 What is the "Union" type and how is it used in TypeScript?

The "Union" type in TypeScript allows a variable to have multiple possible types. It is represented using the pipe symbol (|) between the types.

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Q.63 How do you define and use literal types in TypeScript?

Literal types in TypeScript allow you to specify exact values that a variable can have. They are represented by using a specific value as the type.

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Q.64 What are "Type Aliases" in TypeScript?

Type Aliases in TypeScript allow you to create a new name for an existing type. They provide a way to give descriptive names to complex or lengthy type definitions. Type Aliases are created using the type keyword.

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Q.65 What are "Type Guards" in TypeScript?

Type Guards in TypeScript allow you to narrow down the type of a variable within a conditional block. They are used to perform runtime checks on types and conditionally execute code based on the type. Type Guards can be implemented using the typeof, instanceof, or custom type predicates.

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Q.66 What is an "Intersection" type in TypeScript?

An "Intersection" type in TypeScript allows you to combine multiple types into a single type that has all the properties and methods of the intersected types. It is represented using the ampersand (&) symbol between the types.

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Q.67 How do you use "Mapped Types" in TypeScript?

Mapped Types in TypeScript allow you to transform the properties of an existing type to create a new type. They provide a way to iterate over the keys of a type and create a modified version of it. Mapped Types are defined using the keyof operator and the in keyword.

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Q.68 How does "Conditional Types" work in TypeScript?

Conditional Types in TypeScript allow you to define types that depend on a condition. They provide a way to perform type inference and conditional type mapping. Conditional Types are defined using the extends keyword and conditional expressions.

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Q.69 What is the "keyof" operator in TypeScript?

The keyof operator in TypeScript is used to obtain the union of all the keys of a given type. It allows you to extract the names of properties or methods from an object type. The resulting type is a union of string literals representing the keys.

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Q.70 What are generics in TypeScript?

Generics in TypeScript allow you to create reusable components or functions that can work with multiple types. They provide a way to define placeholders for types that are specified when the component or function is used, enabling type safety and flexibility.

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Q.71 How do you declare a generic type in TypeScript?

To declare a generic type in TypeScript, you can use angle brackets (<>) followed by a type parameter name. This type parameter can be used as a placeholder for a specific type.

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Q.72 What is the purpose of using generics?

The purpose of using generics is to create reusable code that can work with different types without sacrificing type safety. Generics enable the creation of components and functions that can adapt to various data types while preserving the integrity of the types involved.

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Q.73 How can you use generics with arrays in TypeScript?

You can use generics with arrays in TypeScript to ensure type safety when working with array elements. For example, you can create an array of a specific type using a generic array notation: const numbers: Array = [1, 2, 3, 4, 5]; In this example, the Array notation specifies that the numbers array should only contain numbers.

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Q.74 How can you create a generic class in TypeScript?

To create a generic class in TypeScript, you can define the class with one or more generic type parameters. These parameters can then be used within the class to represent different types.

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Q.75 What are the benefits of using generics in TypeScript?

The benefits of using generics in TypeScript include: Increased type safety: Generics enable the compiler to catch type errors at compile time, reducing the likelihood of runtime errors. Code reusability: Generics allow you to create components and functions that can work with multiple types, promoting code reuse and reducing duplication. Flexibility: Generics provide flexibility by allowing consumers of generic components to choose the specific types they need, making the code more adaptable.

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Q.76 How can you constrain the types used in generics?

You can constrain the types used in generics by specifying constraints using the extends keyword. This ensures that the generic type satisfies certain criteria.

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Q.77 What is the difference between generics and any in TypeScript?

Generics provide type safety and preserve type information at compile time, whereas any allows for dynamic typing and bypasses type checking. Generics enable you to work with multiple types in a type-safe manner, while any sacrifices type safety by essentially turning off type checking for a particular value or expression.

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Q.78 How can you specify default types for generics in TypeScript?

You can specify default types for generics in TypeScript by using the = sign followed by a default type. This default type is used when the type argument is not explicitly provided.

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Q.79 How can you create a generic interface in TypeScript?

To create a generic interface in TypeScript, you can declare the interface with one or more generic type parameters. These type parameters can be used within the interface to define the types of properties or methods.

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