TypeScript: Inconsistent Type Narrowing When Using Optional Chaining with Array Methods
I'm trying to implement I'm working on a personal project and I've been banging my head against this for hours..... I'm working with an scenario with type narrowing when using optional chaining in TypeScript, specifically when combined with array methods. I have a function that processes user data, and it sometimes returns `undefined` for specific users. The question arises when I try to filter out users who have a specific property. Here’s the relevant code snippet: ```typescript interface User { id: number; name: string; age?: number; } const users: User[] = [ { id: 1, name: 'Alice' }, { id: 2, name: 'Bob', age: 25 }, { id: 3, name: 'Charlie' } ]; const getUsersWithAge = (users: User[]): User[] => { return users.filter(user => user.age !== undefined); }; const result = getUsersWithAge(users); console.log(result); ``` While I expect `result` to contain only users who have the `age` property, TypeScript seems to be losing track of the type after applying the `filter` method, as it still allows for `undefined`. When I try to access `result[0].age`, I get the following behavior: ``` Property 'age' does not exist on type 'User'. ``` I’ve tried explicitly asserting the type within the filter function, like so: ```typescript const result = getUsersWithAge(users).map(user => user.age as number); ``` But this still gives me the same behavior. I also ensured that `strictNullChecks` is enabled in my `tsconfig.json`. Does anyone have insights on how to handle this type narrowing correctly in this case, or is it a limitation in TypeScript's type inference with optional chaining and array methods? I’m using TypeScript version 4.5.2. Thanks in advance! My development environment is Ubuntu. How would you solve this? I'm using Typescript 3.11 in this project.