Debugging gets faster when you stop guessing. First reproduce the issue consistently, then narrow the failure to one layer: markup, styling, JavaScript logic, data, or network.
Read the first error first
Later errors can be side effects. Start with the earliest console error, inspect its file and line, and confirm the values entering that code.
Reduce the problem
Temporarily remove unrelated code and test the smallest version that still fails. A focused reproduction is easier for you—and an AI assistant—to reason about.