👀 Views: 81 💬 Answers: 1 📅 Created: 2025-06-13

typescript type-system intersection-types TypeScript

Does anyone know how to I'm building a feature where I'm migrating some code and Hey everyone, I'm running into an issue that's driving me crazy. I'm working with a type behavior while trying to use intersection types with union types in my TypeScript function parameters. Specifically, I want to create a function that accepts either an object with properties `a` and `b`, or an object that has a property `c`, all while ensuring that the function can handle a combination of these types. My current function signature looks like this: ```typescript type TypeA = { a: number; b: number; }; type TypeB = { c: string; }; type CombinedType = TypeA | TypeB; type IntersectionType = CombinedType & { d: boolean; }; function processInput(input: IntersectionType) { console.log(input.d); if ('a' in input) { console.log(input.a); } else if ('c' in input) { console.log(input.c); } } ``` However, when I try to call `processInput` like this: ```typescript processInput({ a: 1, b: 2, d: true }); processInput({ c: 'test', d: false }); processInput({ a: 1, c: 'test', d: true }); // This should be valid but gives an behavior ``` I get the following TypeScript behavior: ``` Argument of type '{ a: number; c: string; d: boolean; }' is not assignable to parameter of type 'IntersectionType'. Object literal may only specify known properties, and 'c' does not exist in type 'TypeA'. ``` It seems like TypeScript is not allowing the combination of properties from both types within the intersection. I have tried using type assertions, but that seems to defeat the purpose of type safety. Is there a better way to structure these types to allow for this behavior, or is there a fundamental scenario with how I’m combining intersection and union types? This is part of a larger service I'm building. I'm working in a Windows 10 environment. Am I approaching this the right way? This is part of a larger application I'm building. Any ideas how to fix this? I'm working with Typescript in a Docker container on Windows 10. What are your experiences with this? I recently upgraded to Typescript 3.9.