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Generic Type Builder

Compose generic TypeScript types from labeled fields instead of fighting angle brackets. Add type parameters with optional extends constraints and = defaults, pick a shape — interface, type alias, mapped type, conditional type, or a generic function — and the builder emits the declaration, a realistic usage example that instantiates it with concrete arguments, and a plain-English explanation of every parameter. Everything is live and captured in a shareable URL.

About this ToolHow it works, benefits & use cases

Generic type signatures get noisy fast once you stack constraints and defaults, and a single misplaced angle bracket breaks the whole thing. This builder assembles a generic declaration from labeled fields instead of raw text, so the syntax is always correct. Add as many type parameters as you need, each with an optional extends constraint and an optional = default, then pick a shape: a generic interface or type alias with object fields, a mapped type with readonly / optional modifiers (readonly, ?, -readonly, -?), a conditional type with infer-friendly branches, or a generic function signature. As you edit, the tool emits the declaration, a realistic usage example that instantiates the type with concrete arguments (and a second call that relies on your defaults), and a plain-English explanation of what each parameter accepts. A library of ready-made presets — ApiResponse, Paginated, Result, DeepPartial, ReadonlyProps, Mutable, Nullable, ElementOf, and a generic identity function — loads a full structured model in one click, and the entire configuration lives in a shareable URL. Everything runs in the browser; nothing is uploaded.

How to Use

  1. 1Enter a Type name (e.g. ApiResponse) and choose a Kind: interface, type alias, mapped type, conditional type, or generic function.
  2. 2Add type parameters; for each, optionally fill the extends constraint and the = default fields.
  3. 3Fill in the body for that kind — object fields for interface/alias, the value expression and modifiers for a mapped type, the four branches for a conditional, or the parameters and return type for a function.
  4. 4Read the live declaration in the sticky panel, plus the usage example and the per-parameter explanations.
  5. 5Or click a preset chip (ApiResponse, Paginated, Result, DeepPartial, and more) to load a complete model instantly.
  6. 6Copy the output or share the URL — it captures the full builder state.

Key Benefits

  • Builds five shapes from one model: interface, type alias, mapped type, conditional type, and generic function
  • Type parameters carry optional extends constraints and = defaults across every shape
  • Mapped types support readonly / -readonly and ? / -? modifiers with a custom value expression
  • Conditional types expose check / extends / true / false branches for infer patterns
  • Generates a realistic usage example that instantiates the type with concrete arguments
  • Explains, in plain English, what each type parameter accepts and its default
  • Preset library loads complete structured models: ApiResponse, Paginated, Result, DeepPartial, and more
  • Shareable URL captures the entire configuration; nothing is uploaded

Common Use Cases

  • Authoring a constrained response wrapper like ApiResponse<T, E extends Error = Error>
  • Building mapped-type helpers such as DeepPartial, ReadonlyProps, or Mutable without retyping the boilerplate
  • Writing conditional types like ElementOf<T> that extract an array element with infer
  • Designing a Paginated<T> or Result<T, E> shape with the correct fields and parameters
  • Sketching a generic function signature with a constrained type parameter and a typed return
  • Teaching generics by seeing the assembled signature, a concrete instantiation, and an explanation side by side

Generated declaration

export type ApiResponse<T, E = Error> = {
  success: true;
  data: T;
};
2params0constrained1defaults2fields

Usage example

// Usage
type Example = ApiResponse<string, number>;

// Relying on the default for the trailing parameter(s):
type ExampleWithDefaults = ApiResponse<string>;

Type parameters

  • T — accepts any type
  • E — accepts any type; defaults to `Error` when omitted

Presets

Declaration

Type parameters

Object fields

Why a structured builder

Generic signatures get noisy fast once you stack constraints and defaults. Editing labeled fields instead of raw text means you never drop an angle bracket, and the builder reflects every change instantly into the declaration, a concrete usage example, and a per-parameter explanation.

Five shapes, one model

Switch the Kind to emit a generic interface or type alias (object fields), a mapped type with readonly / optional modifiers, a conditional type with infer-friendly branches, or a generic function signature — the type parameters carry across all of them.

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