Cannot define an arrow function with a single generic type parameter that has a default value with JSX

[giladgo]

Bug Report

🔎 Search Terms

typescript
jsx
tsx
generic
arrow function
default value

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics.

⏯ Playground Link

Playground link with relevant code

💻 Code

import React from 'react'

const myFunc = <T = string,>() => {

}

🙁 Actual behavior

The typescript compiler throws multiple errors

🙂 Expected behavior

I would be allowed to create an arrow function with a single type parameter that has a default value.

Note:

• This happens only on with JSX enabled
• Removing the default value (= string) doesn't throw errors
• Adding a second type parameter before T, ie <K, T=string> doesn't throw errors

[MartinJohns]

Duplicate of Related: #15713.

Workaround: T extends any = string

[RyanCavanaugh]

We should lookahead one token here for = the same way we do for ,

[giladgo]

Amazing! thanks :)

[florianmatz]

I'm to dumb to use it. Gilad Goldberg (@giladgo) Could you provide a example syntax?

[MartinJohns]

Florian Matz (@florianmatz) What do you mean? This works now: const myFunc = <T = string>() => { }

Make sure you're using TypeScript 4.6.2.

[AndreaEsposit]

I am experiencing the same problem on Typescript 5.1.3. Is it just me?