As far as I know, there are only three countries that do not have slums.
Singapore
In Singapore, housing is handled by the Housing and Development Board (HDB), which provides flats (similar to Indonesian flats) to ordinary Singaporeans. So, no one can build slums at will.
Flats are a solution for Singapore where land is very limited and premium class property prices are very expensive for most Singaporeans to afford.
The lease term for a flat in Singapore is 99 years and can be extended by the family members of the original tenant, usually the children and grandchildren of the previous tenant.
Monaco
A haven for a select few species known as "bears," this small country in southern France, with the same flag as Indonesia, has long been known as a tax haven. It's no surprise that many wealthy individuals, professional athletes, and celebrities choose to reside in Monaco.
Monaco is also a favorite holiday destination on the Mediterranean coast. It's one of the host countries for Formula One and one of the routes for the legendary Tour de France cycling race. It's also the center of casino gambling in Europe.
In short, there are no poor people here.
Vatican
The official residence of the Pope and the center of Catholicism worldwide. The population is limited, and most of them are monks devoted to God. They live in buildings provided by the church for hundreds of years.