Typescript function type order issue with |

Question

I just needed to type an argument of one of my functions. Not a very uncommon task, but this time it could be two types, a string or a function. So I tried

function listen (event: string | () => void): void {}

Example

This gives an error. Typescript doesn't understand anymore whats going on.

However, one way to fix this is to swap them

 function listen (event: () => void | string): void {}

Well I guess it makes sense. Can someone explain to me whats going on here or point me to the right documentation

() => void | string means () => (void | string). You want (() => void) | string or string | (() => void). — p.s.w.g
– p.s.w.g, Commented Jul 25, 2019 at 21:05

SLaks · Accepted Answer · 2019-07-25 21:03:13Z

2

It looks like, inside a type expression, the parser expects ( to open a group only.

Wrap the function type in parentheses and it will work.

answered Jul 25, 2019 at 21:03

SLaks

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