deepkit/type for TypeScript: Complete Guide with Examples

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In the TypeScript world, there is a fundamental problem: types exist only at compile time. After transpilation to JavaScript, all type information disappears. This makes runtime validation, dynamic serialization, or building complex systems dependent on type metadata impossible. The deepkit/type library solves this problem radically — it brings the TypeScript type system into runtime, allowing you to work with types as data.

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The main value of deepkit/type is that it does not require writing duplicate schemas (as in Zod or Yup). You describe types once — in pure TypeScript — and the library automatically extracts them at runtime. This enables data validation, serialization/deserialization, reflection (obtaining information about properties, generics, decorators), and even generating code based on types. deepkit/type is not just a validator, but a full-fledged framework for metaprogramming.

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The library is ideal for projects where strict typing is important at all levels: API servers (validating incoming requests), databases (mapping types to SQL), configuration files, RPC systems, and code generation tools. Thanks to integration with deepkit/framework, it is also used to create high-performance server applications with automatic validation and serialization.

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Installation

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deepkit/type is an npm package for TypeScript. Installation is done with the standard command:

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npm install @deepkit/type reflect-metadata

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You will also need to configure tsconfig.json to support decorators and emit metadata:

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{\n  "compilerOptions": {\n    "experimentalDecorators": true,\n    "emitDecoratorMetadata": true,\n    "strict": true,\n    "target": "ES2020",\n    "module": "commonjs",\n    "outDir": "./dist"\n  }\n}

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If you are using ts-node to run, make sure it loads reflect-metadata first. Add the following to the beginning of your entry file:

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import 'reflect-metadata';

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Quick Start

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A minimal example demonstrates how to declare a type and get its runtime representation:

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import 'reflect-metadata';\nimport { typeOf, ReflectionClass } from '@deepkit/type';\n\n// Declare an interface\ninterface User {\n  id: number;\n  name: string;\n  email?: string;\n}\n\n// Get the runtime type\nconst userType = typeOf();\nconsole.log(userType); // ReflectionClass\n\n// Check properties\nconst reflection = ReflectionClass.from(userType);\nconsole.log(reflection.getProperty('name').type); // ReflectionProperty\nconsole.log(reflection.getProperty('id').type.kind); // 'number'\n

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This code will output information about the User type to the console. Note: we did not write any schemas — only a pure TypeScript interface.

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Main Methods and Functions

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Below are the key features of deepkit/type. For each method, the signature, a brief description, and a working example are provided.

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1. typeOf

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Signature: typeOf<T>(): ReflectionClass

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What it does: Extracts the runtime representation of type T. Returns a ReflectionClass object containing complete information about the type's structure: properties, their types, modifiers (optional, readonly), generics, and decorators.

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import { typeOf } from '@deepkit/type';\n\ninterface Product {\n  id: number;\n  title: string;\n  price: number;\n}\n\nconst productType = typeOf();\nconsole.log(productType.getProperty('title').type.kind); // 'string'\nconsole.log(productType.getProperty('price').type.kind); // 'numb