# url.build

## Function

Constructs a URL string from an object configuration.

## Syntax

```bt
url(config).build()
url('').build(config)
```

## Parameters

| Parameters | Type | Required | Default value | Description |
| ------ | ------ | ------ | ------ | ------ |
| config | Object | No | The object passed in when constructing the url | URL configuration object. |

## config object field

| Field | Type | Required | Default value | Description |
| ------ | ------ | ------ | ------ | ------ |
| scheme | String | No | `http` | URL scheme, no need to write `://`. |
| host | String | No | Empty string | Hostname or IP. When there is no host, the URL starting with `scheme://` will still be generated according to the current implementation. |
| port | Int/String | No | None | Port number. If the field value is not `empty` or `null`, it will be converted to a string and spelled after host. |
| path | String | No | `/` | URL path. If it does not start with `/`, `/` will be automatically added. |
| query | Object/String/Any | No | None | Query string. Object will be URL encoded item by key value; String will be used as query string as it is; other values will be converted to string first. Empty strings are not written to `?`. |
| fragment | String | No | None | URL fragment, no need to write `#`; percent encoding will be done when writing. |
| username | String | No | None | Username. If it is not empty, `username@` will be written and percent-encoded. |
| password | String | No | None | Password. Only when `username` is non-empty, it will be written and percent-encoded. |

## Return value

| Type | Description |
| ------ | ------ |
| String | The constructed URL. |

## Code Example

```bt
result = url({scheme: 'https', host: 'btlang.org', path: '/docs', query: {q: 'BT Lang'}}).build()

// Output: https://btlang.org/docs?q=BT%20Lang
print result
```

## Notes

- When `query` is Object, field names and values will be encoded according to URL component rules.
- `build()` only does lightweight string construction and does not check whether host is empty or whether scheme actually exists.