Typed argument forwarding
Derive the argument tuple for a wrapper or forwarding function.
TypeScript · Type Alias
Parameters
Parameters<Type> creates a tuple containing the parameter types of a function type. It is useful for reusing or transforming a function's argument list without manually duplicating it.
- Since
- 3.1
How it works
Parameters<Type> evaluates a function type's parameter list as a tuple. Applying it to a function declaration via typeof preserves the parameter names and optionality represented by that function type; for overloaded functions, TypeScript uses the last signature.
Syntax
Parameters<Type>Parameters
| Name | Type | Description |
|---|---|---|
Type | function type | A function type whose parameter types should be extracted. |
Returns
A tuple containing the parameter types of Type.
Key points
- Parameters converts a function type's parameter types into a tuple type.
- No import is needed because Parameters is a globally available utility type.
- The utility was introduced in TypeScript 3.1.
- For an overloaded function, the result comes from its last signature.
Common mistakes
- With overloads, Parameters uses only the final overload signature, so earlier overload parameter lists are not represented.
Example: Extracting a function's parameter tuple
function greet(name: string, age?: number): string {
return age === undefined ? `Hello, ${name}` : `Hello, ${name}, age ${age}`;
}
type GreetParameters = Parameters<typeof greet>;
const args: GreetParameters = ["Ada", 36];
greet(...args);The tuple type captures the parameter list of greet, including its optional second parameter.
When to use it
Reusable test and adapter types
Keep mock, spy, or adapter arguments synchronized with an existing function.
Function type transformations
Build APIs that transform a function while preserving its parameter list.
Related
ReturnTypeConstructorParametersInstanceTypeInferring Within Conditional Types