Repository: TypeScript
Issue: No type error when setting an unknown property of a JSX element
Labels: Suggestion,In Discussion

# Bug Report

I've augmented `solid-js`'s `JSX.IntrinsicElements` with new intrinsic elements made within my solid-js-consuming application.

When I write an unknown prop manually (not with spread as detailed in https://github.com/microsoft/TypeScript/issues/18801), I do not get a type error.


https://user-images.githubusercontent.com/297678/104986944-4a9c7c00-59c9-11eb-9a24-24edb1dd8617.mp4


### 🔎 Search Terms

typescript no error on unknown prop

### 🕗 Version & Regression Information

Not sure when this happened, but I've been trying unsuccessfully to `import {JSX}` into particular files to as not to clash with React's global `JSX` definitions, but because `import {JSX}` does not work (https://github.com/microsoft/TypeScript/issues/41813), I've since started trying to use the new `@jsxImportSource` feature as comments inside my files.

So far, I have gotten types to appear using per-file `@jsxImportSource` comments, and there are no other global JSX types in those files (I have segregated React JSX code from Solid.js JSX code using Project References and placing the two forms of files in separate projects with their own tsconfig to ensure only certain types are available to each group of files).

So, I'm not sure, but it could be that I'm one of the few people testing unknown waters in TypeScript, and hitting this issue.

I have not tried with an older version of TypeScript, because `@jsxImportSource` doesn't work there, and `import {JSX}` has never worked for JSX markup (#41813).


### ⏯ Playground Link

I tried to recreate the issue in playground, but due to https://github.com/microsoft/TypeScript-Website/issues/1427, `@jsxImportSource` comments to not work in the playground, so I can't reproduce a small version of my environment.

The following is my attempt to try `@jsxImportSource` just to get started, but no luck (same example from https://github.com/microsoft/TypeScript-Website/issues/1427):

[playground](https://www.typescriptlang.org/play?jsx=1#code/PQKgBAAgVgzgHgSQLYAcD2AnALgZTQVwwGMBTMGNAGwEsATAWljBGAChXrVMswBvAKRwANADRgiGEgEMsJAKIAzBSSJYAvmAUY0SMAHIKNBrD3tgwMABUAFtRhg7m6gDsSYw3Ub2ScO1hgAdKxEaM4wPERgALzikjLySipYZhY2jo4KLmRYaGhizmhgJBjaGEFYAJ4o2STh0WCCQgEIzlgYLjDURHKUJEgkrTApYHIlmABcDcJFvf2tDqg0RNRYlBVg1lL2ldX6Us4VemAARipS+DBkBQ6txQpSpPqNza3tYV09fQP+Rz5+gcFQnVaPUADy0agANzAwAAfGBWEA)

I'll see if I can make a reproduction in a repo...

### 💻 Code

As you see in the video, I can hover on an unknown prop, and TypeScript will infer the type of the prop as whatever value I have set it to.

For example, in the next video I changed the value to a number, and now the type shows up as a number instead of a function:


https://user-images.githubusercontent.com/297678/104987468-a9aec080-59ca-11eb-83ed-11f802b4d8ba.mp4

I have double checked that the type supplied to `JSX.IntrinsicElements` does not contain an index signature. There are only specific properties defined in the type of the video's `timeline-scrubber`.

### 🙁 Actual behavior

It infers unknown props.

### 🙂 Expected behavior

It should give a type error.