# Variable Resolvers

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

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

Note that if your variables are in a namespace (e.g. `$myVars:var`), you must use the second or third
type as the plain object implementation does not support namespaces.

## Variable values

You can use any of five types of values to specify the values of variables:

- string
- number
- boolean
- single node (will be treated as a node set)
- array of nodes or array-like collection of nodes (will be treated as a node set)

## Variable Resolver Type 1: Plain object

A plain object with variable names as the keys and variable values as the values.

Example usage:

```typescript
const evaluator = xpath.parse('concat($character1, ", ", $character2, ", and ", $character3)');
const mainCharacters = evaluator.evaluateString({
  variables: {
    character1: 'Harry',
    character2: 'Ron',
    character3: 'Hermione'
  }
});
```

## Variable Resolver Type 2: Function

A function that takes a variable name as its first parameter and an optional namespace URI as its second parameter
and returns a value based on the name and namespace.

Example usage:

```typescript
const evaluator = xpath.parse('concat($hp:character1, ", ", $hp:character2, ", and ", $hp:character3)');
const mainCharacters = evaluator.evaluateString({
  variables: function(name: string, namespace: string) {
    if (namespace === 'http://sample.org/harrypotter/') {
      switch (name) {
        case 'character1':
          return 'Harry';
        case 'character2':
          return 'Ron';
        case 'character3':
          return 'Hermione';
      }
    }
  },
  namespaces: {
    hp: 'http://sample.org/harrypotter/'
  }
});
```

## Function Resolver Type 3: Object with `getFunction` method

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

Example usage:

```typescript
const evaluator = xpath.parse('concat($hp:character1, ", ", $hp:character2, ", and ", $hp:character3)');
const mainCharacters = evaluator.evaluateString({
  variables: {
    getVariable(name: string, namespace: string) {
      if (namespace === 'http://sample.org/harrypotter/') {
        switch (name) {
          case 'character1':
            return 'Harry';
          case 'character2':
            return 'Ron';
          case 'character3':
            return 'Hermione';
        }
      }
    }
  },
  namespaces: {
    hp: 'http://sample.org/harrypotter/'
  }
});
```