feat: allow use as a singleton (#413)

Adds a singleton version of VerifiedFetch to make it even easier to get started:

```TypeScript
import { verifiedFetch } from '@helia/verified-fetch'

const response = await verifiedFetch('ipfs://Qmfoo')

console.info(await response.json())
```

---------

Co-authored-by: Russell Dempsey <1173416+SgtPooki@users.noreply.github.com>
Co-authored-by: Daniel Norman <1992255+2color@users.noreply.github.com>

Author: 3 people

README.md

@@ -13,56 +13,49 @@
 
 # About
 
-`@helia/verified-fetch` is a library that provides a fetch-like API for fetching trustless content from IPFS and verifying it.
+`@helia/verified-fetch` provides a [fetch](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API)-like API for retrieving content from the [IPFS](https://ipfs.tech/) network.
 
-This library should act as a replacement for the `fetch()` API for fetching content from IPFS, and will return a [`Response`](https://developer.mozilla.org/en-US/docs/Web/API/Response) object that can be used in a similar manner to the `fetch()` API. This means browser and HTTP caching inside browser main threads, web-workers, and service workers, as well as other features of the `fetch()` API should work in a way familiar to developers.
+All content is retrieved in a [trustless manner](https://www.techopedia.com/definition/trustless), and the integrity of all bytes are verified by comparing hashes of the data.
 
-Exports a `createVerifiedFetch` function that returns a `fetch()` like API method Helia for fetching IPFS content.
+This is a marked improvement over `fetch` which offers no such protections and is vulnerable to all sorts of attacks like [Content Spoofing](https://owasp.org/www-community/attacks/Content_Spoofing), [DNS Hijacking](https://en.wikipedia.org/wiki/DNS_hijacking), etc.
+
+A `verifiedFetch` function is exported to get up and running quickly, and a `createVerifiedFetch` function is also available that allows customizing the underlying [Helia](https://helia.io/) node for complete control over how content is retrieved.
+
+Browser-cache-friendly [Response](https://developer.mozilla.org/en-US/docs/Web/API/Response) objects are returned which should be instantly familiar to web developers.
 
 You may use any supported resource argument to fetch content:
 
-- CID instance
+- [CID](https://multiformats.github.io/js-multiformats/classes/cid.CID.html) instance
 - IPFS URL
 - IPNS URL
 
-## Example
+## Example - Getting started
 
 ```typescript
-import { createVerifiedFetch } from '@helia/verified-fetch'
-
-const fetch = await createVerifiedFetch({
- gateways: ['https://mygateway.example.net', 'https://trustless-gateway.link']
-})
+import { verifiedFetch } from '@helia/verified-fetch'
 
-const resp = await fetch('ipfs://bafy...')
+const resp = await verifiedFetch('ipfs://bafy...')
 
 const json = await resp.json()
 ```
 
 ## Example - Using a CID instance to fetch JSON
 
 ```typescript
-import { createVerifiedFetch } from '@helia/verified-fetch'
+import { verifiedFetch } from '@helia/verified-fetch'
 import { CID } from 'multiformats/cid'
 
-const fetch = await createVerifiedFetch({
- gateways: ['https://mygateway.example.net', 'https://trustless-gateway.link']
-})
-
 const cid = CID.parse('bafyFoo') // some image file
-const response = await fetch(cid)
+const response = await verifiedFetch(cid)
 const json = await response.json()
 ```
 
 ## Example - Using IPFS protocol to fetch an image
 
 ```typescript
-import { createVerifiedFetch } from '@helia/verified-fetch'
+import { verifiedFetch } from '@helia/verified-fetch'
 
-const fetch = await createVerifiedFetch({
- gateways: ['https://mygateway.example.net', 'https://trustless-gateway.link']
-})
-const response = await fetch('ipfs://bafyFoo') // CID for some image file
+const response = await verifiedFetch('ipfs://bafyFoo') // CID for some image file
 const blob = await response.blob()
 const image = document.createElement('img')
 image.src = URL.createObjectURL(blob)
@@ -71,23 +64,43 @@ document.body.appendChild(image)
 
 ## Example - Using IPNS protocol to stream a big file
 
+```typescript
+import { verifiedFetch } from '@helia/verified-fetch'
+
+const response = await verifiedFetch('ipns://mydomain.com/path/to/very-long-file.log')
+const bigFileStreamReader = await response.body.getReader()
+```
+
+## Configuration
+
+### Custom HTTP gateways and routers
+
+Out of the box `@helia/verified-fetch` uses a default set of [trustless gateways](https://specs.ipfs.tech/http-gateways/trustless-gateway/) for fetching blocks and [HTTP delegated routers](https://specs.ipfs.tech/routing/http-routing-v1/) for performing routing tasks - looking up peers, resolving/publishing [IPNS](https://docs.ipfs.tech/concepts/ipns/) names, etc.
+
+It's possible to override these by passing `gateways` and `routers` keys to the `createVerifiedFetch` function:
+
+## Example - Configuring gateways and routers
+
 ```typescript
 import { createVerifiedFetch } from '@helia/verified-fetch'
 
 const fetch = await createVerifiedFetch({
- gateways: ['https://mygateway.example.net', 'https://trustless-gateway.link']
+ gateways: ['https://trustless-gateway.link'],
+ routers: ['http://delegated-ipfs.dev']
 })
-const response = await fetch('ipns://mydomain.com/path/to/very-long-file.log')
-const bigFileStreamReader = await response.body.getReader()
+
+const resp = await fetch('ipfs://bafy...')
+
+const json = await resp.json()
 ```
 
-### Configuration
+### Usage with customized Helia
 
-#### Usage with customized Helia
+For full control of how `@helia/verified-fetch` fetches content from the distributed web you can pass a preconfigured Helia node to `createVerifiedFetch`.
 
-You can see variations of Helia and js-libp2p configuration options at <https://helia.io/interfaces/helia.index.HeliaInit.html>.
+The [helia](https://www.npmjs.com/package/helia) module is configured with a libp2p node that is suited for decentralized applications, alternatively [@helia/http](https://www.npmjs.com/package/@helia/http) is available which uses HTTP gateways for all network operations.
 
-The `@helia/http` module is currently in-progress, but the init options should be a subset of the `helia` module's init options. See <https://github.com/ipfs/helia/issues/289> for more information.
+You can see variations of Helia and js-libp2p configuration options at <https://helia.io/interfaces/helia.index.HeliaInit.html>.
 
 ```typescript
 import { trustlessGateway } from '@helia/block-brokers'
@@ -111,28 +124,28 @@ const resp = await fetch('ipfs://bafy...')
 const json = await resp.json()
 ```
 
-### Comparison to fetch
+## Comparison to fetch
 
-First, this library will require instantiation in order to configure the gateways and delegated routers, or potentially a custom Helia instance. Secondly, once your verified-fetch method is created, it will act as similar to the `fetch()` API as possible.
+This module attempts to act as similarly to the `fetch()` API as possible.
 
 [The `fetch()` API](https://developer.mozilla.org/en-US/docs/Web/API/fetch) takes two parameters:
 
 1. A [resource](https://developer.mozilla.org/en-US/docs/Web/API/fetch#resource)
 2. An [options object](https://developer.mozilla.org/en-US/docs/Web/API/fetch#options)
 
-#### Resource argument
+### Resource argument
 
-This library intends to support the following methods of fetching web3 content from IPFS:
+This library supports the following methods of fetching web3 content from IPFS:
 
 1. IPFS protocol: `ipfs://<cidv0>` & `ipfs://<cidv0>`
 2. IPNS protocol: `ipns://<peerId>` & `ipns://<publicKey>` & `ipns://<hostUri_Supporting_DnsLink_TxtRecords>`
 3. CID instances: An actual CID instance `CID.parse('bafy...')`
 
-As well as support for pathing & params for item 1&2 above according to [IPFS - Path Gateway Specification](https://specs.ipfs.tech/http-gateways/path-gateway) & [IPFS - Trustless Gateway Specification](https://specs.ipfs.tech/http-gateways/trustless-gateway/). Further refinement of those specifications specifically for web-based scenarios can be found in the [Web Pathing Specification IPIP](https://github.com/ipfs/specs/pull/453).
+As well as support for pathing & params for item 1 & 2 above according to [IPFS - Path Gateway Specification](https://specs.ipfs.tech/http-gateways/path-gateway) & [IPFS - Trustless Gateway Specification](https://specs.ipfs.tech/http-gateways/trustless-gateway/). Further refinement of those specifications specifically for web-based scenarios can be found in the [Web Pathing Specification IPIP](https://github.com/ipfs/specs/pull/453).
 
-If you pass a CID instance, we assume you want the content for that specific CID only, and do not support pathing or params for that CID.
+If you pass a CID instance, it assumes you want the content for that specific CID only, and does not support pathing or params for that CID.
 
-#### Options argument
+### Options argument
 
 This library does not plan to support the exact Fetch API options object, as some of the arguments don't make sense. Instead, it will only support options necessary to meet [IPFS specs](https://specs.ipfs.tech/) related to specifying the resultant shape of desired content.
 
@@ -177,15 +190,15 @@ Some in-flight specs (IPIPs) that will affect the options object this library su
 4. [IPIP-0328: JSON and CBOR Response Formats on HTTP Gateways](https://specs.ipfs.tech/ipips/ipip-0328/)
 5. [IPIP-0288: TAR Response Format on HTTP Gateways](https://specs.ipfs.tech/ipips/ipip-0288/)
 
-#### Response types
+### Response types
 
 This library's purpose is to return reasonably representable content from IPFS. In other words, fetching content is intended for leaf-node content -- such as images/videos/audio & other assets, or other IPLD content (with link) -- that can be represented by <https://developer.mozilla.org/en-US/docs/Web/API/Response#instance_methods>. The content type you receive back will depend upon the CID you request as well as the `Accept` header value you provide.
 
 All content we retrieve from the IPFS network is obtained via an AsyncIterable, and will be set as the [body of the HTTP Response](https://developer.mozilla.org/en-US/docs/Web/API/Response/Response#body) via a [`ReadableStream`](https://developer.mozilla.org/en-US/docs/Web/API/Streams_API/Using_readable_streams#consuming_a_fetch_as_a_stream) or other efficient method that avoids loading the entire response into memory or getting the entire response from the network before returning a response to the user.
 
 If your content doesn't have a mime-type or an [IPFS spec](https://specs.ipfs.tech), this library will not support it, but you can use the [`helia`](https://github.com/ipfs/helia) library directly for those use cases. See [Unsupported response types](#unsupported-response-types) for more information.
 
-##### Handling response types
+#### Handling response types
 
 For handling responses we want to follow conventions/abstractions from Fetch API where possible:
 
@@ -194,12 +207,12 @@ For handling responses we want to follow conventions/abstractions from Fetch API
 - For plain text in utf-8, you would call `.text()`
 - For streaming response data, use something like `response.body.getReader()` to get a [`ReadableStream`](https://developer.mozilla.org/en-US/docs/Web/API/Streams_API/Using_readable_streams#consuming_a_fetch_as_a_stream).
 
-##### Unsupported response types
+#### Unsupported response types
 
 - Returning IPLD nodes or DAGs as JS objects is not supported, as there is no currently well-defined structure for representing this data in an [HTTP Response](https://developer.mozilla.org/en-US/docs/Web/API/Response). Instead, users should request `aplication/vnd.ipld.car` or use the [`helia`](https://github.com/ipfs/helia) library directly for this use case.
 - Others? Open an issue or PR!
 
-#### Response headers
+### Response headers
 
 This library will set the [HTTP Response](https://developer.mozilla.org/en-US/docs/Web/API/Response) headers to the appropriate values for the content type according to the appropriate [IPFS Specifications](https://specs.ipfs.tech/).
 
@@ -209,13 +222,13 @@ Some known header specifications:
 - <https://specs.ipfs.tech/http-gateways/trustless-gateway/#response-headers>
 - <https://specs.ipfs.tech/http-gateways/subdomain-gateway/#response-headers>
 
-#### Possible Scenarios that could cause confusion
+### Possible Scenarios that could cause confusion
 
-##### Attempting to fetch the CID for content that does not make sense
+#### Attempting to fetch the CID for content that does not make sense
 
 If you request `bafybeiaysi4s6lnjev27ln5icwm6tueaw2vdykrtjkwiphwekaywqhcjze`, which points to the root of the en.wikipedia.org mirror, a response object does not make sense.
 
-#### Errors
+### Errors
 
 Known Errors that can be thrown: