# useEvaluation for a value, not just a boolean, and a safe default if it fails

Asked by Hannah Schmidt on 2026-10-05. Tags: react, useevaluation, cql.

Most examples I find use `useEvaluation` for an on/off flag. I want a component to
read an actual value from an evaluation instead, like the visitor's country or a
computed tier, and branch on that.

My concern is the failure case. If the query cannot resolve for some reason, I do
not want the component to throw and take the UI down. What is the right way to
give it a safe default?

## 2 answers

### Accepted answer from dermot_k (2026-10-07)

`useEvaluation` runs a CQL query and returns its result, and that result can be
any value, not only a boolean. So a query like `location's country` gives you
the country back directly:

```tsx
const country = useEvaluation<string>("location's country", {
    fallback: 'US',
});
```

The `fallback` is exactly your safety net: if the evaluation cannot resolve, the
hook resolves to that default instead of throwing, so the UI keeps rendering.
CQL is evaluated in real time per interaction and targets under 20ms, so this is
not a slow path, but the fallback covers the case where something does go wrong.

### Answer from helena.dev (2026-10-07)

One behaviour to keep in mind while you branch on the value: undefined variables
evaluate to `false` by design. So a condition that reads an attribute the
visitor does not have yet resolves to false rather than erroring, which is
usually what you want, but it means you should guard the not-yet-known case
explicitly rather than assuming a missing value throws.

#### Reply from Hannah Schmidt (2026-10-08)

That covers my case exactly. Value out of the query plus a fallback, and I
will handle the undefined branch on purpose. Thanks.