These can be passed in an object to useAsync(options), or as props to <Async {...options}> and custom instances.

• ​promise An already started Promise instance.

• ​promiseFn Function that returns a Promise, automatically invoked.

• ​deferFn Function that returns a Promise, manually invoked with run.

• ​watch Watch a value and automatically reload when it changes.

• ​watchFn Watch this function and automatically reload when it returns truthy.

• ​initialValue Provide initial data or error for server-side rendering.

• ​onResolve Callback invoked when Promise resolves.

• ​onReject Callback invoked when Promise rejects.

• ​onCancel Callback invoked when a Promise is cancelled.

• ​reducer State reducer to control internal state updates.

• ​dispatcher Action dispatcher to control internal action dispatching.

• ​debugLabel Unique label used in DevTools.

• ​suspense Enable experimental Suspense integration.

useFetch additionally takes these options:

• ​defer Force the use of deferFn or promiseFn.

• ​json Enable JSON parsing of the response.

promise

Promise

A Promise instance which has already started. It will simply add the necessary resolve/reject callbacks and set startedAt to the time promise was first provided. Changing the value of promise will cancel any pending promise and listen to the new one. If promise is initially undefined, the React Async state will be pending.

Note that reload will not do anything when using promise. Use promiseFn instead.

promiseFn

function(props: Object, controller: AbortController): Promise

A function that returns a promise. It is automatically invoked in componentDidMount and componentDidUpdate. The function receives all component props (or options) and an AbortController instance as arguments.

Be aware that updating promiseFn will trigger it to cancel any pending promise and load the new promise. Passing an inline (arrow) function will cause it to change and reload on every render of the parent component. You can avoid this by defining the promiseFn value outside of the render method. If you need to pass variables to the promiseFn, pass them as additional props to <Async>, as promiseFn will be invoked with these props. Alternatively you can use useCallback or memoize-one to avoid unnecessary updates.

deferFn

function(args: any[], props: Object, controller: AbortController): Promise

A function that returns a promise. This is invoked only by manually calling run(...args). Any arguments to run are passed-through as an array via args, so you can pass data through either args or props, as needed. The deferFn is commonly used to send data to the server following a user action, such as submitting a form. You can use this in conjunction with promiseFn to fill the form with existing data, then updating it on submit with deferFn.

Be aware that when using both promiseFn and deferFn, the shape of their fulfilled value should match, because they both update the same data.

watch

any

Watches this property through componentDidUpdate and re-runs the promiseFn when the value changes, using a simple reference check (oldValue !== newValue). If you need a more complex update check, use watchFn instead.

watchFn

function(props: Object, prevProps: Object): boolean | any

Re-runs the promiseFn when this callback returns truthy (called on every update). Any default props specified by createInstance are available too.

initialValue

any | Error

Initial state for data or error (if instance of Error); useful for server-side rendering. When an initialValue is provided, the promiseFn will not be invoked on first render. Instead, status will be immediately set to fulfilled or rejected and your components will render accordingly. If you want to trigger the promiseFn regardless, you can call reload() or use the watch or watchFn option.

Note that onResolve or onReject is not invoked in this case and no promise prop will be created.

onResolve

function(data: any): void

Callback function invoked when a promise resolves, receives data as argument.

onReject

function(reason: Error): void

Callback function invoked when a promise rejects, receives rejection reason (error) as argument.

onCancel

function(): void

Callback function invoked when a promise is cancelled, either manually using cancel() or automatically due to props changes or unmounting.

reducer

function(state: any, action: Object, internalReducer: function(state: any, action: Object))

State reducer to take full control over state updates by wrapping the internal reducer. It receives the current state, the dispatched action and the internal reducer. You probably want to invoke the internal reducer at some point.

This is a power feature which loosely follows the state reducer pattern. It allows you to control state changes by intercepting actions before they are handled, or by overriding or enhancing the reducer itself.

dispatcher

function(action: Object, internalDispatch: function(action: Object), props: Object)

Action dispatcher to take full control over action dispatching by wrapping the internal dispatcher. It receives the original action, the internal dispatcher and all component props (or options). You probably want to invoke the internal dispatcher at some point.

This is a power feature similar to the state reducer pattern. It allows you to control state changes by intercepting actions before they are dispatched, to dispatch additional actions, possibly later in time.

debugLabel

string

A unique label to describe this React Async instance, used in React DevTools (through useDebugValue) and React Async DevTools.

suspense

boolean

Enables experimental Suspense integration. This will make React Async throw a promise while loading, so you can use Suspense to render a fallback UI, instead of using <IfPending>. Suspense differs in 2 main ways:

• <Suspense> should be an ancestor of your Async component, instead of a descendant. It can be anywhere up in the

  component hierarchy.

• You can have a single <Suspense> wrap multiple Async components, in which case it will render the fallback UI until

  all promises are settled.

Note that the way Suspense is integrated right now may change. Until Suspense for data fetching is officially released, we may make breaking changes to its integration in React Async in a minor or patch release. Among other things, we'll probably add a cache of sorts.

defer

boolean

Enables the use of deferFn if true, or enables the use of promiseFn if false. By default this is automatically chosen based on the request method (deferFn for POST / PUT / PATCH / DELETE, promiseFn otherwise).

json

boolean

Enables or disables JSON parsing of the response body. By default this is automatically enabled if the Accept header is set to "application/json".