The SonarLint tool was used to analyze data and identify the top 5 most common TypeScript issues, with the fifth issue being optional property declarations. Optional object properties can be declared using either a union type or an optional property syntax, but both approaches have different implications for how the properties should be used in the code. The use of both `?` and `undefined` syntax is considered confusing and should be avoided, as it communicates nothing to other developers about how the interface should be used. To avoid this issue, developers can default to using the optional property syntax and use union types with caution. This lint rule will alert developers if they make a mistake in their code. The analysis also highlights the importance of choosing one approach over the other when declaring optional object properties.