Source has X element(s) but target allows only 1

I had the same problem. The braces have to go after the type:

const FooMapping: [{id: FooOperation, display: string}] 

should be

const FooMapping: {id: FooOperation, display: string}[] 

Easy way to remember this is to replace the object with a type and see if it makes sense:

[string] doesn’t work but string[] does