{"@context":"https://schema.org","@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is a generic in TypeScript?","acceptedAnswer":{"@type":"Answer","text":"A generic allows creating reusable components that work with multiple types rather than a single one. Generics use type parameters (like T) to represent any type. This enables writing type-safe code without sacrificing flexibility, for example a function that manipulates arrays of any type while preserving type information."}},{"@type":"Question","name":"What is the role of the Partial utility type?","acceptedAnswer":{"@type":"Answer","text":"Partial transforms all properties of a type T into optional properties. This is particularly useful for update functions where only certain fields need to be modified. For example, to update a user, it's possible to provide only the name without having to specify all other fields. TypeScript ensures that only existing properties can be passed."}},{"@type":"Question","name":"What is the difference between Pick and Omit?","acceptedAnswer":{"@type":"Answer","text":"Pick selects only properties K from a type T, creating a new type with only those properties. Conversely, Omit excludes properties K from a type T, creating a new type with all properties except those specified. Pick is useful for creating partial views with necessary fields, while Omit is practical for creating types without certain sensitive fields like passwords."}},{"@type":"Question","name":"How to constrain a generic to certain types?","acceptedAnswer":{"@type":"Answer","text":"Use the extends keyword to define a constraint on a generic. For example, T extends string means that T can only be string or a subtype of string. This ensures that the passed type has certain properties or methods while maintaining generic flexibility. Constraints can also be interfaces or union types to limit accepted types."}},{"@type":"Question","name":"Which utility type makes all properties required?","acceptedAnswer":{"@type":"Answer","text":"Required transforms all optional properties of a type T into required properties. It's the opposite of Partial. This type is useful for ensuring all properties of an object are present, for example during data validation or creation of complete entities. TypeScript generates compilation errors if a property is missing."}},{"@type":"Question","name":"What is the purpose of the Record utility type?","acceptedAnswer":{"@type":"Answer","text":"Record creates an object type with keys of type K and values of type T. It's a shortcut for defining an object type where all keys have the same value type. For example, Record represents an object with string keys and number values. It's particularly useful for creating dictionaries, maps, or configurations with a homogeneous structure."}},{"@type":"Question","name":"What is a mapped type in TypeScript?","acceptedAnswer":{"@type":"Answer","text":"A mapped type transforms each property of an existing type by applying an operation. It uses the syntax [P in keyof T] to iterate over the keys of type T. Mapped types allow creating new types by modifying existing properties, such as making all properties readonly, optional, or changing their type. Utility types like Partial, Required and Readonly are implemented using mapped types."}},{"@type":"Question","name":"What is the role of the infer keyword in conditional types?","acceptedAnswer":{"@type":"Answer","text":"The infer keyword allows extracting and capturing a type in a conditional clause. It's used in conditional types to automatically infer a type from another type without having to specify it explicitly. For example, it's possible to extract the return type of a function or the element type of an array. Infer makes conditional types more powerful and flexible."}},{"@type":"Question","name":"What is the syntax for creating a conditional type?","acceptedAnswer":{"@type":"Answer","text":"A conditional type uses the syntax T extends U ? X : Y, similar to the ternary operator in JavaScript. If type T is assignable to type U, the resulting type is X, otherwise it's Y. Conditional types allow creating dynamic types based on conditions, which is particularly useful for creating advanced types and custom utility types."}},{"@type":"Question","name":"What is a decorator in TypeScript?","acceptedAnswer":{"@type":"Answer","text":"A decorator is a special function that can be attached to a class, method, property, or parameter to modify or annotate its declaration. Decorators use the @expression syntax and are evaluated at declaration time. They are widely used in Angular for features like @Component, @Injectable, or @Input. Decorators allow adding metadata and behaviors without directly modifying the code."}},{"@type":"Question","name":"What is the difference between ReturnType and Parameters?","acceptedAnswer":{"@type":"Answer","text":"ReturnType extracts the return type of a function T, while Parameters extracts parameter types as a tuple. For example, for a function (a: string, b: number) => boolean, ReturnType returns boolean and Parameters returns [string, number]. These utility types are useful for creating derived types from existing function signatures without duplication."}},{"@type":"Question","name":"How to make all properties of a type readonly?","acceptedAnswer":{"@type":"Answer","text":"Use the Readonly utility type to transform all properties of a type T into readonly properties. This prevents property modification after object initialization. Readonly is particularly useful for creating immutable objects and ensuring no accidental modifications occur. TypeScript generates compilation errors if a modification attempt is detected."}},{"@type":"Question","name":"What is the main advantage of namespaces in TypeScript?","acceptedAnswer":{"@type":"Answer","text":"Namespaces allow organizing code into logical groups and avoiding name collisions in the global scope. They create a container that encapsulates variables, functions, classes and interfaces. Although ES6 modules are now preferred for code organization, namespaces remain useful for structuring code in a single file or for legacy libraries."}},{"@type":"Question","name":"What is the difference between import type and import normal?","acceptedAnswer":{"@type":"Answer","text":"import type imports only types that will be erased during compilation, while a normal import also imports executable JavaScript code. Using import type ensures no runtime code is imported, which can reduce bundle size and avoid circular imports. It also clarifies code intent by explicitly indicating that only typing information is imported."}},{"@type":"Question","name":"How to create a type that excludes null and undefined?","acceptedAnswer":{"@type":"Answer","text":"Use the NonNullable utility type to create a type that excludes null and undefined from a type T. This is particularly useful when working with potentially nullable values and wanting to guarantee a value exists. NonNullable is often used after null checks to narrow the type and avoid null reference errors."}},{"@type":"Question","name":"What is keyof in TypeScript?","acceptedAnswer":{"@type":"Answer","text":"keyof is an operator that creates a union type containing all keys of an object type. For example, keyof { name: string, age: number } produces the type 'name' | 'age'. It's a fundamental tool for creating advanced types and mapped types. keyof ensures a key exists on an object and is widely used in utility types and generic constraints."}},{"@type":"Question","name":"What is the difference between a namespace and an ES6 module?","acceptedAnswer":{"@type":"Answer","text":"ES6 modules use import/export and create isolated scope per file, while namespaces use the namespace syntax and can span multiple files. ES6 modules are the modern standard and offer better tree-shaking, asynchronous loading and JavaScript ecosystem compatibility. Namespaces are considered legacy but remain useful for internal TypeScript code without external dependencies."}},{"@type":"Question","name":"How to create a generic with multiple type parameters?","acceptedAnswer":{"@type":"Answer","text":"Declare multiple type parameters separated by commas between angle brackets, like Function or Map. Each type parameter can have its own constraints with extends. Multi-parameter generics allow creating complex type relationships, like a function that transforms type T to type U, or a data structure with keys K and values V."}},{"@type":"Question","name":"What is the Exclude utility type?","acceptedAnswer":{"@type":"Answer","text":"Exclude creates a type by removing from T all types that are assignable to U. For a union type, it filters out members that match U. For example, Exclude<'a' | 'b' | 'c', 'a'> produces 'b' | 'c'. It's the opposite of Extract which only keeps assignable types. Exclude is useful for creating subsets of union types by excluding certain values."}},{"@type":"Question","name":"In what order are decorators applied to a method?","acceptedAnswer":{"@type":"Answer","text":"Decorators are evaluated from top to bottom but applied from bottom to top. If multiple decorators are present, the decorator closest to the method executes first, then moves upward. For example, with @A @B @C method(), the evaluation order is A, B, C but the application order is C, B, A. This composition allows creating chains of modifications where each decorator transforms the result of the previous one."}}]}