René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Suggestion] Override and identifier! for Mapped Type and extend interface","articleBody":"\u003c!-- BUGS: Please use this template. --\u003e\r\n\u003c!-- QUESTIONS: This is not a general support forum! Ask Qs at http://stackoverflow.com/questions/tagged/typescript --\u003e\r\n\u003c!-- SUGGESTIONS: See https://github.com/Microsoft/TypeScript-wiki/blob/master/Writing-Good-Design-Proposals.md --\u003e\r\n\r\n**TypeScript Version:**  2.4.0 / nightly (2.5.0-dev.201xxxxx)\r\n2.5.2\r\n\r\n**Situation**\r\nThere are lots of demand of more flexibility to Mapped Type. For example:\r\n1. Extend from a type and override some fields\r\n1. Turn optional field to required\r\n1. Only make some specific fields to optional\r\n\r\n**Suggestion**\r\n1. **Support multiple field definition in one type, the latter definition (same fields) will overwrite the previous**\r\n1. **Support identifier! for type definition which will remove `null` and `undefined`.**\r\n\r\n**Code**\r\n\r\n```ts\r\ntype Full = {\r\n  field1: string;\r\n  field2: string;\r\n  ....\r\n  fieldN: string;\r\n}\r\n\r\n//Suggestion 1: Multiple fields in type definition and support override\r\ntype SomeOptional = {\r\n  [P in keyof Full]: Full[P];  // extends\r\n  [P in 'field1' | 'field2']? : Full[P];  // override field1 and field2 to optional\r\n  xxx: string;  // and can add more fields.\r\n}\r\n\r\n//Suggestion 2: Identifer! to remove null and undefined\r\ntype SomeRequired = {\r\n  [P in keyof SomeOptional]: SomeOptional[P];  // extends\r\n  field1: SomeOptional[field1]!;  // ! will remove null and undefined from a type\r\n}\r\n```\r\n\r\n**Benifit**\r\n1. It can resolve most problem with required and optional in current open issues. (like #15012, #16173, #13224, #12578, )\r\n2. More flexible to extend and override, in limited complexity.\r\n\r\n\r\nBTW, `function getProperty` in #12578 is a solution, but it cannot be used in declaration files.(`.d.ts`)","author":{"url":"https://github.com/k8w","@type":"Person","name":"k8w"},"datePublished":"2017-09-19T05:53:57.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/18567/TypeScript/issues/18567"}