An XIDError whose code and details are narrowed to one code (or, with the default argument, the union over every code), so error.code === "Duplicate" narrows error.details.item to a string.
XIDError
code
details
error.code === "Duplicate"
error.details.item
An
XIDErrorwhosecodeanddetailsare narrowed to one code (or, with the default argument, the union over every code), soerror.code === "Duplicate"narrowserror.details.itemto a string.