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Extract breaks when upgrading to TypeScript 5.1 #54680
Description
Activity
Bisects to #53098.
As a workaround you can use this:
export function handle<M extends Message>(callbacks: { - [K in M["type"]]: (msg: Extract<M, { type: K }>["value"]) => unknown; + [K in M["type"]]: (msg: (M & { type: K })["value"]) => unknown; }) { window.addEventListener("message", (event) => { const msg = event.data as M; callbacks[msg.type as keyof typeof callbacks](msg.value); }); }Reacted by Yuri SulymaI think the new behavior is defensible. The old behavior was too permissive and would for example allow
callbacks[msg.type as keyof typeof callbacks](42); callbacks[msg.type as keyof typeof callbacks]("hello");
even though one or the other must be wrong.
I think this is an instance of the pattern that was addressed in #47109. I'd suggest rewriting the example to
type MessageMap = { A: string, B: number } type Message<K extends keyof MessageMap> = { type: K, value: MessageMap[K] }; type Callbacks<K extends keyof MessageMap> = { [P in K]: (value: MessageMap[P]) => unknown }; export function handle<K extends keyof MessageMap>(callbacks: Callbacks<K>) { window.addEventListener("message", (event) => { const msg = event.data as Message<K>; callbacks[msg.type](msg.value); }); }
It kinda means that the workaround that I suggested here is broken, right? I mean, if this
Extract-based solution is considered to be flawed (I understand the argument behind it) then the intersection variant should really be treated the same way.This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
- locked as resolved and limited conversation to collaborators
on Oct 22, 2025
Bug Report
Some of my code using
Extractstopped working when I upgraded from 5.0 to 5.1. This may be related to #54676. I didn't see anything in the release announcement to suggest that this might happen.🔎 Search Terms
Extract, TypeScript 5.1, discriminated union
🕗 Version & Regression Information
⏯ Playground Link
https://typescript-eslint.io/play/#ts=5.1.3&sourceType=module&code=C4TwDgpgBAglC8UDeAoKVSQFxQEQ1wG40oA3AQwBsBXCHAZ2ACcBLAOwHNiBfYlTaACEEyEgJy5BREhRp0obagFsARhCY8+AqAFkI9euQ7REcAD5RBfCAA8wAeybAoAM2psAxsBb22UABbkbAAmlBAAPDpQtsAQIfS6+obGAHwAFB5UlCrkHgDW9Dio6ADaANJQ7LoluAK4ALr1OGlK9Bw4AKI2zLnAkQA0yBjg8hXcKTWytA0AlAgpUO55bPYA7mw8c8VQq+zBawB05MHBHaRxwAAyLIxx6mm4SklGELiDaRDnbMBz8Avb6A8vkYUFaHBEnwuB2C5GA5Cg5ASOmI6EBWRy+XoJTBB20iKgeQgIHsLmGkBJUEylGyuQK9RabQOUwgMxRUG4rJQ3BQQA&eslintrc=N4KABGBEBOCuA2BTAzpAXGYBfEWg&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA
💻 Code
🙁 Actual behavior
In TypeScript 5.0, this code works. In TypeScript 5.1, it complains about
msg.value, with the error🙂 Expected behavior
Code should work as it did in 5.0.4. If this is now intended behavior, I would like to know how to fix my code.