# Namespace Resolvers

The methods on the [XPathEvaluator](XPathEvaluator.md) type can optionally take a namespace resolver to resolve
namespace references in the XPath expression being evaluated.

There are three ways to specify a namespace resolver and you can use any one of them depending on which is
most suited to your particular situation.

## Namespace Resolver Type 1: Plain object

A plain object with namespace prefixes as the keys and namespace URIs as the values:

Example usage:

```typescript
const evaluator = xpath.parse('/bk:book/hp:characters');
const characters = evaluator.select({
  node: myBookNode,
  namespaces: {
    bk: 'http://sample.org/books/',
    hp: 'http://sample.org/harrypotter/'
  }
});
```

## Namespace Resolver Type 2: Function

A function that takes a namespace prefix as a parameter and returns the corresponding namespace URI.

Example usage:

```typescript
const evaluator = xpath.parse('/bk:book/hp:characters');
const characters = evaluator.select({
  node: myBookNode,
  namespaces(prefix: string) {
    if (prefix === 'bk') {
      return 'http://sample.org/books/';
    }
    if (prefix === 'hp') {
      return 'http://sample.org/books/';
    }
  }
});
```

## Namespace Resolver Type 3: Object with `getNamespace` method

An object with a method named `getNamespace` that works in the same way as the function-based namespace resolver
described above.

Example usage:

```typescript
const evaluator = xpath.parse('/bk:book/hp:characters');
const characters = evaluator.select({
  node: myBookNode,
  namespaces: {
    getNamespace(prefix: string) {
      if (prefix === 'bk') {
        return 'http://sample.org/books/';
      }
      if (prefix === 'hp') {
        return 'http://sample.org/books/';
      }
    }
  }
});
```