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GeezerVison is showing github.com — Object.assign return assigned type instead merged by anion155 · Pull Request #28553 · microsoft/TypeScript

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@anion155

@anion155 Anton Mikhailov (anion155) commented Nov 15, 2018

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Got 'assing' in my eyes. Found some old bugs created in 16 (#11100). So I made this quick PR.

Situation:

let a = {
  a: 'string',
  b: { c: 4 },
};
let b = {
  a: 5,
  b: { d: 3 },
}
Object.assign({ }, a, b);

Result of this operation is { a: 5, b: { d: 3 } }, but TS says its of { a: string & number; b: { c: number; d: number; } } type, which is obviously wrong (according to mozilla's JS reference).

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@anion155
Object.assign return assigned type instead merged
777a252
@sandersn

Nathan Shively-Sanders (sandersn) commented Nov 19, 2018

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We actually decided recently to double down on this incorrect definition of spread and Object.assign in #28234. In practise, the most common use of spread is to spread two identical types. The second most common use is to spread two disjoint types. Spreading two overlapping types is pretty rare, so we decided not to create a type operator for it.

Also note that Assign is still not correct, since it doesn't understand own properties, optional properties, or private properties. getSpreadType handles these aspects, but doesn't work for higher-order cases since we didn't create the spread type operator.
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@sandersn Nathan Shively-Sanders (sandersn) closed this Nov 19, 2018
@jcalz Joe Calzaretta (jcalz) mentioned this pull request Jun 19, 2019
Bug Report + Proposal: Object.assign() #31982
Closed
@karlhorky Karl Horky (karlhorky) mentioned this pull request Jul 4, 2019
Full Handbook Page on Soundness microsoft/TypeScript-Handbook#1059
Open
@microsoft Microsoft (microsoft) locked as resolved and limited conversation to collaborators Oct 21, 2025
@typescript-bot

TypeScript Bot (typescript-bot) commented Oct 21, 2025

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This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.
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3 participants

@anion155 @sandersn @typescript-bot
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