A pure democracy cannot function except in very small jurisdictions. It would involve the concept of one person, one vote on every issue that comes up, and would not require a constitution.
But without a constitution, elected officials, once in office, can change the rules, declare themselves to be dictators for life, steal all of the peoples' money, use the military to take away basic human rights. The list could go on, but a recognizable democracy would not long survive without the constitution.
In the US, we have a constitution that is the envy of the world; most other constitutions are modeled at least partially on the US experience. But an understanding of US history, especially the condition of the states at the time the constitution was ratified, would show that without a strong constitution, the US as a single nation would not have survived its infancy, and could never have become a world power.