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perf(types): prevent immediate deep and circular resolution for deep mock #123

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Aug 3, 2023
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perf(types): prevent immediate deep and circular resolution for deep mock #123

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perf(types): prevent immediate deep and circular resolution for deep …
millsp Aug 2, 2023
7cb9815
make it work with private properties
millsp Aug 2, 2023
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37 changes: 25 additions & 12 deletions src/Mock.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -30,22 +30,35 @@ export interface CalledWithMock<T, Y extends any[]> extends jest.Mock<T, Y> {
calledWith: (...args: Y | MatchersOrLiterals<Y>) => jest.Mock<T, Y>;
}

export type MockProxy<T> = {
[K in keyof T]: T[K] extends (...args: infer A) => infer B ? CalledWithMock<B, A> : T[K];
} &
T;
export type _MockProxy<T> = {
[K in keyof T]: T[K] extends (...args: infer A) => infer B
? T[K] & CalledWithMock<B, A>
: T[K];
};

export type MockProxy<T> = _MockProxy<T> & T;

export type DeepMockProxy<T> = {
export type _DeepMockProxy<T> = {
// This supports deep mocks in the else branch
[K in keyof T]: T[K] extends (...args: infer A) => infer B ? CalledWithMock<B, A> : DeepMockProxy<T[K]>;
} &
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This causes TypeScript to expand and immediately compute the type, potentially doing dangerous eager loading on deep and recursive or mutually self-referencing objects - potentially causing perf issues.

T;
[K in keyof T]: T[K] extends (...args: infer A) => infer B
? T[K] & CalledWithMock<B, A>
: T[K] & _DeepMockProxy<T[K]>;
};

export type DeepMockProxyWithFuncPropSupport<T> = {
// we intersect with T here instead of on the mapped type above to
// prevent immediate type resolution on a recursive type, this will
// help to improve performance for deeply nested recursive mocking
// at the same time, this intersection preserves private properties
export type DeepMockProxy<T> = _DeepMockProxy<T> & T;
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@millsp millsp Aug 3, 2023 •
edited
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Suggested change
export type DeepMockProxy<T> = _DeepMockProxy<T> & T;
export type DeepMockProxy<T> = _DeepMockProxy<T> & T;

In theory re-intersecting should not be necessary because we did already in the mapped types. But because the original type can contain private fields, using a mapped type on it erases that information. So the solution is to re-intersect once only.


export type _DeepMockProxyWithFuncPropSupport<T> = {
// This supports deep mocks in the else branch
[K in keyof T]: T[K] extends (...args: infer A) => infer B ? CalledWithMock<B, A> & DeepMockProxy<T[K]> : DeepMockProxy<T[K]>;
} &
T;
[K in keyof T]: T[K] extends (...args: infer A) => infer B
? CalledWithMock<B, A> & DeepMockProxy<T[K]>
: DeepMockProxy<T[K]>;
};

export type DeepMockProxyWithFuncPropSupport<T> = _DeepMockProxyWithFuncPropSupport<T> & T;
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I did all of them for consistency.


export interface MockOpts {
deep?: boolean;
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