Document the FileOutput and wait parameters (#326)

This commit takes a first pass at documenting the `FileOutput` and `wait` parameters introduced in 1.0.0 as well as calling out various potential gotchas with the API around URLs and data-uris based on feedback from the issues.

README.md

@@ -64,6 +64,15 @@ console.log(output.url()); // 'https://replicate.delivery/pbxt/GtQb3Sgve42ZZyVnt
 console.log(output.blob()); // Blob
 ```
 
+> [!NOTE]
+> A model that outputs file data returns a `FileOutput` object by default. This is an implementation
+> of `ReadableStream` that returns the file contents. It has a `.blob()` method for accessing a
+> `Blob` representation and a `.url()` method that will return the underlying data-source.
+>
+> **This data source can be either a remote URL or a data-uri with base64 encoded data. Check
+> out the documentation on [creating a prediction](https://replicate.com/docs/topics/predictions/create-a-prediction)
+> for more information.**
+
 You can also run a model in the background:
 
 ```js
@@ -277,6 +286,7 @@ const replicate = new Replicate(options);
 | `options.baseUrl`              | string   | Defaults to https://api.replicate.com/v1                                                                                         |
 | `options.fetch`                | function | Fetch function to use. Defaults to `globalThis.fetch`                                                                            |
 | `options.fileEncodingStrategy` | string   | Determines the file encoding strategy to use. Possible values: `"default"`, `"upload"`, or `"data-uri"`. Defaults to `"default"` |
+| `options.useFileOutput`        | boolean  | Determines if the `replicate.run()` method should convert file output into `FileOutput` objects |
 
 
 The client makes requests to Replicate's API using
@@ -333,16 +343,23 @@ const output = await replicate.run(identifier, options, progress);
 | ------------------------------- | -------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `identifier`                    | string   | **Required**. The model version identifier in the format `{owner}/{name}:{version}`, for example `stability-ai/sdxl:8beff3369e81422112d93b89ca01426147de542cd4684c244b673b105188fe5f`                       |
 | `options.input`                 | object   | **Required**. An object with the model inputs.                                                                                                                                                              |
-| `options.wait`                  | object   | Options for waiting for the prediction to finish                                                                                                                                                            |
-| `options.wait.interval`         | number   | Polling interval in milliseconds. Defaults to 500                                                                                                                                                           |
+| `options.wait`                  | object   | Options for waiting for the prediction to finish                        | 
+| `options.wait.type`             | `"poll" \| "block"`   | `"block"` holds the request open, `"poll"` makes repeated requests to fetch the prediction. Defaults to `"block"`  |
+| `options.wait.interval`         | number   | Polling interval in milliseconds. Defaults to 500 |
+| `options.wait.timeout`          | number   | In `"block"` mode determines how long the request will be held open until falling back to polling. Defaults to 60 |
 | `options.webhook`               | string   | An HTTPS URL for receiving a webhook when the prediction has new output                                                                                                                                     |
 | `options.webhook_events_filter` | string[] | An array of events which should trigger [webhooks](https://replicate.com/docs/webhooks). Allowable values are `start`, `output`, `logs`, and `completed`                                                    |
 | `options.signal`                | object   | An [AbortSignal](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) to cancel the prediction                                                                                                     |
 | `progress`                      | function | Callback function that receives the prediction object as it's updated. The function is called when the prediction is created, each time it's updated while polling for completion, and when it's completed. |
 
 Throws `Error` if the prediction failed.
 
-Returns `Promise<object>` which resolves with the output of running the model.
+Returns `Promise<unknown>` which resolves with the output of running the model.
+
+> [!NOTE]
+> Currently the TypeScript return type of `replicate.run()` is `Promise<object>` this is
+> misleading as a model can return array types as well as primative types like strings,
+> numbers and booleans.
 
 Example:
 
@@ -364,6 +381,26 @@ const onProgress = (prediction) => {
 const output = await replicate.run(model, { input }, onProgress)
 ```
 
+#### Sync vs. Async API (`"poll"` vs. `"block"`)
+
+The `replicate.run()` API takes advantage of the [Replicate sync API](https://replicate.com/docs/topics/predictions/create-a-prediction)
+which is optimized for low latency requests to file models like `black-forest-labs/flux-dev` and 
+`black-forest-labs/flux-schnell`. When creating a prediction this will hold a connection open to the
+server and return a `FileObject` containing the generated file as quickly as possible.
+
+> [!NOTE]
+> In this mode the `url()` method on the `FileObject` may refer to either a remote URL or
+> base64 encoded data-uri. The latter is an optimization we make on certain models to deliver
+> the files faster to the client.
+>
+> If you need the prediction URLs for whatever reason you can opt out of the sync mode by
+> passing `wait: { "type": "poll" }` to the `run()` method.
+>
+> ```js
+> const output = await replicate.run(model, { input, wait: { type: "poll" } });
+> output.url() // URL<https://...>
+> ```
+
 ### `replicate.stream`
 
 Run a model and stream its output. Unlike [`replicate.prediction.create`](#replicatepredictionscreate), this method returns only the prediction output rather than the entire prediction object.
@@ -1192,6 +1229,41 @@ The `replicate.request()` method is used by the other methods
 to interact with the Replicate API.
 You can call this method directly to make other requests to the API.
 
+### `FileOutput`
+
+`FileOutput` is a `ReadableStream` instance that represents a model file output. It can be used to stream file data to disk or as a `Response` body to an HTTP request.
+
+```javascript
+const [output] = await replicate.run("black-forest-labs/flux-schnell", { 
+  input: { prompt: "astronaut riding a rocket like a horse" }
+});
+
+// To access the file URL (or data-uri):
+console.log(output.url()); //=> "http://example.com"
+
+// To write the file to disk:
+fs.writeFile("my-image.png", output);
+
+// To stream the file back to a browser:
+return new Response(output);
+
+// To read the file in chunks:
+for await (const chunk of output) {
+  console.log(chunk); // UInt8Array
+}
+```
+
+You can opt out of FileOutput by passing `useFileOutput: false` to the `Replicate` constructor:
+
+```javascript
+const replicate = new Replicate({ useFileOutput: false });
+```
+
+| method               | returns   | description                                                  |
+| -------------------- | ------    | ------------------------------------------------------------ |
+| `url()`              | string    | A `URL` object representing the HTTP URL or data-uri         |
+| `blob()`             | object    | A `Blob` instance containing the binary file                 |
+
 ## Troubleshooting
 
 ### Predictions hanging in Next.js