I'm working on a TypeScript project where I have a generic action handler that can return either a success or error response. The issue is that TypeScript isn't properly inferring the return type in some cases.
Code Example
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2type ActionResponse<T> = SuccessResponse<T> | ErrorResponse;
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4
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6type SuccessResponse<T> = {
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8 success: true;
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10 data: T;
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12};
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14
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16type ErrorResponse = {
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18 success: false;
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20 error: { message: string };
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22 status: number;
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24};
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26
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28async function getQuestions(params: SearchParams): Promise<ActionResponse<Question[]>> {
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30 // ... implementation
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32}
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2type ActionResponse<T> = SuccessResponse<T> | ErrorResponse;
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4
5
6type SuccessResponse<T> = {
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8 success: true;
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10 data: T;
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12};
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14
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16type ErrorResponse = {
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18 success: false;
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20 error: { message: string };
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22 status: number;
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24};
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26
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28async function getQuestions(params: SearchParams): Promise<ActionResponse<Question[]>> {
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30 // ... implementation
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32}
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Problem
When I call this function, TypeScript sometimes doesn't narrow the type properly:
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2const result = await getQuestions(params);
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4if (result.success) {
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6 // TypeScript should know result.data exists here
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8 console.log(result.data); // Type error: Property 'data' does not exist
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10}
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2const result = await getQuestions(params);
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4if (result.success) {
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6 // TypeScript should know result.data exists here
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8 console.log(result.data); // Type error: Property 'data' does not exist
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10}
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Question
How can I improve the type definitions to ensure proper type narrowing with discriminated unions in async functions?
