TypeScript Type Playground

You wrote type Result = Pick<User, "id" | "name"> & { role: "admin" | "user" } and need to see what the resolved type looks like — which fields it has, which are optional, what the final shape is. The TypeScript compiler shows errors but doesn’t “expand” complex types into their resolved form. You need a playground that resolves and displays the final type structure.

Why This Playground (Not the TypeScript Utility Types Reference)

PureDevTools has a TypeScript Utility Types Reference for learning what each utility type does. This playground lets you write TypeScript type definitions and see the resolved output — define interfaces, apply utility types (Pick, Omit, Partial, Required, conditional types, mapped types), and see the expanded result instantly. Use the reference to learn; use this playground to experiment.

What Is a TypeScript Type Playground?

A TypeScript type playground lets you experiment with TypeScript’s type system without setting up a compiler or IDE. This tool is a client-side type resolver — it parses TypeScript type definitions you enter, applies utility type transformations, and shows you the fully expanded, resolved type.

It is designed for developers learning the TypeScript type system, exploring how utility types work, and quickly checking how complex type operations transform an interface.

Supported Type Operations

Utility Types (TypeScript Standard Library)

TypeScript ships with a set of built-in utility types that transform other types. This tool fully resolves them:

Union and Intersection Types

Union types (A | B | C) represent values that can be one of several types. The resolver flattens nested unions and deduplicates identical members.

Intersection types (A & B) merge multiple types together. When two object types are intersected, the resolver merges their properties into a single object type.

Conditional Types

Conditional types follow the pattern T extends U ? TrueType : FalseType. The resolver evaluates basic conditional types and distributes them over union types.

Example:

type IsString<T> = T extends string ? "yes" : "no";
type Result = IsString<string | number>;
// Resolved: "yes" | "no"

Mapped Types

Mapped types iterate over a set of keys to construct new object types:

type Flags<Keys extends string> = {
  [K in Keys]: boolean;
};

type ThemeFlags = Flags<"dark" | "compact" | "rtl">;
// Resolved: { dark: boolean; compact: boolean; rtl: boolean; }

The optional (?) and readonly modifiers in mapped types are also supported.

keyof Operator

The keyof operator produces a union of all property name string literals from an object type:

interface User {
  id: number;
  name: string;
  email: string;
}

type UserKeys = keyof User;
// Resolved: "id" | "name" | "email"

Template Literal Types

Template literal types combine string literal types using backtick syntax:

type EventName<T extends string> = `on${Capitalize<T>}`;
type ClickEvent = EventName<"click">;
// Resolved: "onClick"

Indexed Access Types

Access a specific property type from an object type using bracket notation:

interface Config {
  host: string;
  port: number;
}
type HostType = Config["host"];
// Resolved: string

Common Use Cases

API response modeling: Define your API response interface and use Partial<T> for optional patch requests, Pick<T, K> to select only the fields you need.

Form handling: Use Partial<UserFormData> to make all form fields optional while keeping the interface as the source of truth.

Discriminated unions: Explore how Extract and Exclude can filter union types to specific variants.

Understanding generics: Instantiate generic types with concrete arguments and see what the resolved type looks like.

Type derivation: Derive child types from a parent interface using utility types instead of duplicating property definitions.

This Tool vs. the Full TypeScript Compiler

This tool is a pattern-based client-side type resolver, not the full TypeScript compiler. It handles the majority of common patterns correctly, but has some limitations:

• No type inference from expressions or values — only type-level declarations
• No import statements — all types must be defined in the same input
• Limited infer support — basic ReturnType and Parameters work, but complex infer patterns may return unresolved
• No overloaded functions — only single-signature function types
• Circular types — recursive types terminate at a depth limit to prevent infinite loops

For precise TypeScript type checking, use the TypeScript Playground (requires internet) or an IDE with TypeScript support.

Frequently Asked Questions

What is the difference between type and interface in TypeScript? Both define the shape of an object. interface is extendable (you can extends it and declaration-merge it). type is an alias and can represent any type expression including unions, intersections, and primitives. For object types, they behave similarly. In this tool, both are resolved the same way.

What does Partial<T> actually do? Partial<T> adds the ? modifier to every property of T, making them all optional. For example, Partial<{ id: number; name: string }> resolves to { id?: number; name?: string }. It’s equivalent to the mapped type { [K in keyof T]?: T[K] }.

How does Pick differ from Omit? Pick<T, K> keeps only the properties listed in K. Omit<T, K> removes the properties listed in K and keeps everything else. Use Pick when you know exactly which fields you want; use Omit when you want to remove a few fields from a larger type.

What is the keyof operator for? keyof T produces a union of all property name string literals in type T. It’s useful as a constraint to ensure a value is a valid key of an object: function get<T, K extends keyof T>(obj: T, key: K): T[K].

Can I use this for runtime code? No. TypeScript types are purely compile-time constructs — they are erased before the JavaScript code runs. This tool only works with type-level code, not runtime values or logic.