tsdoc info lost after bind a methord to an object

Question

I incidentally found:

/** foo is a method **/
let foo = ()=>{}
foo() // here tsdoc work fine if you hover the cursor on it
let obj={foo: foo}
obj.foo() // here tsdoc info is lost

If you visit the foo method via obj.foo(), the tsdoc info will lose. What's the cause of it?