Type Alias Exact<ParameterType, InputType>

Exact: IsEqual<ParameterType, InputType> extends true
    ? ParameterType
    : ParameterType extends unknown[]
        ? Exact<ArrayElement<ParameterType>, ArrayElement<InputType>>[]
        : ParameterType extends readonly unknown[]
            ? ReadonlyArray<
                Exact<ArrayElement<ParameterType>, ArrayElement<InputType>>,
            >
            : ParameterType extends Opaque<infer OpaqueType, infer OpaqueToken>
                ? ParameterType
                : ParameterType extends object
                    ? ExactObject<ParameterType, InputType>
                    : ParameterType

Create a type that does not allow extra properties, meaning it only allows properties that are explicitly declared.

This is useful for function type-guarding to reject arguments with excess properties. Due to the nature of TypeScript, it does not complain if excess properties are provided unless the provided value is an object literal.

Please upvote this issue if you want to have this type as a built-in in TypeScript.

Type Parameters

ParameterType
InputType

Example

type OnlyAcceptName = {name: string};

function onlyAcceptName(arguments_: OnlyAcceptName) {}

// TypeScript complains about excess properties when an object literal is provided.
onlyAcceptName({name: 'name', id: 1});
//=> `id` is excess

// TypeScript does not complain about excess properties when the provided value is a variable (not an object literal).
const invalidInput = {name: 'name', id: 1};
onlyAcceptName(invalidInput); // No errors

Having Exact allows TypeScript to reject excess properties.

Example

import {Exact} from 'type-fest';

type OnlyAcceptName = {name: string};

function onlyAcceptNameImproved<T extends Exact<OnlyAcceptName, T>>(arguments_: T) {}

const invalidInput = {name: 'name', id: 1};
onlyAcceptNameImproved(invalidInput); // Compilation error

Type Alias Exact<ParameterType, InputType>

Type Parameters

Example

Example

Settings