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TypeScript Advanced
Advanced TypeScript — generics, utility types, discriminated unions, and type-level programming.
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What are generics in TypeScript?
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Type parameters that make functions/classes work with any type while preserving type safety. Example: function identity<T>(arg: T): T { return arg; }
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What is the difference between "type" and "interface"?
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Interface: extendable, declaration merging, best for object shapes. Type: can represent unions, intersections, primitives, tuples. Interfaces are often preferred for public APIs.
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What is a discriminated union?
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A union type where each member has a common literal property (discriminant). Enables exhaustive type narrowing in switch statements. Example: type Shape = Circle | Square.
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What is the "infer" keyword?
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Used in conditional types to extract a type. Example: type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never; Infers the return type R.
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What are Partial<T>, Required<T>, Pick<T,K>, Omit<T,K>?
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Utility types: Partial makes all properties optional. Required makes all required. Pick selects specific keys. Omit excludes specific keys.
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What is "as const" assertion?
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Makes values deeply readonly and narrows literal types. const colors = ["red", "blue"] as const; // readonly ["red", "blue"] instead of string[].
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What is a mapped type?
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Creates new types by transforming properties of existing types. Syntax: { [K in keyof T]: NewType }. Built-in examples: Readonly<T>, Record<K,V>.
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What is "satisfies" in TypeScript?
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Validates a value matches a type without widening it. const config = { port: 3000 } satisfies Config; Keeps the literal type while checking compatibility.
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