# Defined test

Learn how to check if a value is defined.

This test checks whether the value of an operand is defined.

An operand is considered defined when it can be resolved to a value, meaning it is not null or undefined. If an operand throws an error, it is considered undefined. For example, if you try to access a property that does not exist, such as a query parameter that does not exist in the current URL, the result is false, not an error.

> **Defined values may still be empty**
>
> This test does not check if the value is empty, only if it is defined. For empty checks, use the [empty test](/reference/cql/expressions/tests/other/empty) instead.

## Syntax

This test has the following syntax:

```cql
/*<value>*/ is defined
```

You can alternatively use the following syntax:

```cql
/*<value>*/ is known
```

Both forms have the same semantics. You can use whichever makes your query more expressive and easier to read.

To negate the test, use the following syntax:

```cql
/*<value>*/ is not defined
```

## Parameters

These are the supported parameters:

- `value`: `any`

  The value to validate.

## Examples

Here is a practical example of the defined test:

```cql
page's query's code is defined
```

The above expression returns true if the current page URL contains a query parameter named `code`, regardless of its value.

You can also use the alternative syntax to achieve the same result:

```cql
page's query's code is known
```