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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Why TDZ checks are not implemented","articleBody":"# Bug Report\r\n\r\nIt may not be a bug, because I see that you clearly know what TDZ check is (such as the code example [in this issue](https://github.com/microsoft/TypeScript/issues/52924#issue-1595962667)), but it is not applied. I did find some related issues reported by TypeScript team members such as https://github.com/microsoft/TypeScript/issues/5174 several years before, but it still passes the check now.\r\n\r\nJust out of curiosity, would you please explain why such checks are not (fully) implemented?\r\n\r\n### 🔎 Search Terms\r\n\r\nTDZ check\r\n\r\n### 🕗 Version \u0026 Regression Information\r\n\r\n\u003c!-- When did you start seeing this bug occur?\r\n\r\n\"Bugs\" that have existed in TS for a long time are very likely to be FAQs; refer to\r\n  https://github.com/Microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs\r\n\r\nIf possible, please try testing the nightly version of TS to see if it's already been fixed.\r\nFor npm: `typescript@next`\r\nThis is also the 'Nightly' version in the playground: http://www.typescriptlang.org/play/?ts=Nightly\r\n\r\nNote: The TypeScript Playground can be used to try older versions of TypeScript.\r\n\r\nPlease keep and fill in the line that best applies:\r\n--\u003e\r\n- This is the behavior in every version I tried, and I reviewed the FAQ for entries.\r\n\r\n### ⏯ Playground Link\r\n\r\n\u003c!--\r\n  A link to a TypeScript Playground \"Share\" link which shows this behavior.\r\n  This should have the same code as the code snippet below, and use whichever settings are relevant to your report.\r\n\r\n  As a last resort, you can link to a repo, but these will be slower for us to investigate.\r\n--\u003e\r\n[Playground link with relevant code](https://www.typescriptlang.org/play?#code/PTAEAsBdIBwZwFwgOYEtLgK4CMB0BjAewFthjV8AnQuQgM0mABUBPGAUwGUrUZHU4cTOzjAArAEYA7ABYAUHUwA7fJFSEloOgAoAHglBLMxbO0oBKUAG85oO6DgB3dPnB7LN+19D4AhnHZQCQRbbzCAG3ZIUF1QAF4ggG5QsL8A0AAmELCw2ISMxNAQB3BCTHCAE1BTUDNqSmr2P0x0+lAmABEALRTQAF85AbliqFhEFHQsPCJScioaekZWDm5KXn5BYVExDIBODJkAYgEhdgBaCTFdq4A2DJubqQVlVXVNNw9eyOjKEXLohLIbTmZJeb4xAxGExmeJBAAMoPsv0gmEoml+QnCkGSvUUKjUGlAQM+OVAyNRml0iLsAz6QA)\r\n\r\n### 💻 Code\r\n\r\n``` typescript\r\n// https://github.com/microsoft/TypeScript/issues/5174\r\nfunction f(x: number) {\r\n    switch(x) {\r\n        case 1:\r\n            let x = 1;\r\n        case 2:\r\n            x = 2; // should be error because of TDZ\r\n    }\r\n}\r\n\r\n// https://github.com/microsoft/TypeScript/issues/52924#issue-1595962667\r\nfunction h() {\r\n    let result = g();\r\n    let x: number = 10;\r\n    return result;\r\n\r\n    function g() {\r\n        return x;\r\n    }\r\n}\r\n```\r\n\r\n### 🙁 Actual behavior\r\n\r\nNo error reported.\r\n\r\n### 🙂 Expected behavior\r\n\r\nTDZ check should be applied.\r\n","author":{"url":"https://github.com/yf-yang","@type":"Person","name":"yf-yang"},"datePublished":"2023-05-22T09:19:05.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/54343/TypeScript/issues/54343"}