In TypeScript, conditional properties allow us to create flexible and type-safe interfaces that adapt based on certain conditions. This is particularly useful when dealing with complex data structures where certain properties should only be present under specific circumstances. In this blog post, we’ll explore how to use conditional properties with a practical example involving reward groups.

Imagine we have a system that manages different types of rewards. Each reward can be of a specific type, such as “FINANCE” or “SHIPPING”.

Depending on the reward type, certain attributes should be included or excluded. For instance, financial rewards should include financial attributes, while shipping rewards should include shipping attributes. Additionally, we want to ensure that certain attributes are only included based on the reward type and reward on conditions.

Conditional Types:
We use a union of three types to handle the conditional properties.

When rewardType is “FINANCE” and rewardOn is “Finance”, financeAttributes is required,
and rewardAttributes and shippingAttributes are not allowed.

When rewardType is “SHIPPING” and rewardOn is not “Finance”, shippingAttributes is required, and financeAttributes is not allowed, but rewardAttributes is included.

For any other rewardType and rewardOn not being “Finance”, rewardAttributes is included, but neither financeAttributes nor shippingAttributes are included.