Placeholder data allows a query to behave as if it already has data, similar to the initialData option, but the data is not persisted to the cache. This comes in handy for situations where you have enough partial (or fake) data to render the query successfully while the actual data is fetched in the background.
Example: An individual blog post query could pull "preview" data from a parent list of blog posts that only include title and a small snippet of the post body. You would not want to persist this partial data to the query result of the individual query, but it is useful for showing the content layout as quickly as possible while the actual query finishes to fetch the entire object.
There are a few ways to supply placeholder data for a query to the cache before you need it:
When we use placeholderData, our Query will not be in a pending state - it will start out as being in success state, because we have data to display - even if that data is just "placeholder" data. To distinguish it from "real" data, we will also have the isPlaceholderData flag set to true on the Query result.
class TodosComponent {
result = injectQuery(() => ({
queryKey: ['todos'],
queryFn: () => fetch('/todos'),
placeholderData: placeholderTodos,
}))
}
class TodosComponent {
result = injectQuery(() => ({
queryKey: ['todos'],
queryFn: () => fetch('/todos'),
placeholderData: placeholderTodos,
}))
}
placeholderData can also be a function, where you can get access to the data and Query meta information of a "previous" successful Query. This is useful for situations where you want to use the data from one query as the placeholder data for another query. When the QueryKey changes, e.g. from ['todos', 1] to ['todos', 2], we can keep displaying "old" data instead of having to show a loading spinner while data is transitioning from one Query to the next. For more information, see Paginated Queries.
class TodosComponent {
result = injectQuery(() => ({
queryKey: ['todos', id()],
queryFn: () => fetch(`/todos/${id}`),
placeholderData: (previousData, previousQuery) => previousData,
}))
}
class TodosComponent {
result = injectQuery(() => ({
queryKey: ['todos', id()],
queryFn: () => fetch(`/todos/${id}`),
placeholderData: (previousData, previousQuery) => previousData,
}))
}
In some circumstances, you may be able to provide the placeholder data for a query from the cached result of another. A good example of this would be searching the cached data from a blog post list query for a preview version of the post, then using that as the placeholder data for your individual post query:
export class BlogPostComponent {
// Until Angular supports signal-based inputs, we have to set a signal
@Input({ required: true, alias: 'postId' })
set _postId(value: number) {
this.postId.set(value)
}
postId = signal(0)
queryClient = injectQueryClient()
result = injectQuery(() => ({
queryKey: ['blogPost', this.postId()],
queryFn: () => fetch(`/blogPosts/${this.postId()}`),
placeholderData: () => {
// Use the smaller/preview version of the blogPost from the 'blogPosts'
// query as the placeholder data for this blogPost query
return queryClient
.getQueryData(['blogPosts'])
?.find((d) => d.id === this.postId())
},
}))
}
export class BlogPostComponent {
// Until Angular supports signal-based inputs, we have to set a signal
@Input({ required: true, alias: 'postId' })
set _postId(value: number) {
this.postId.set(value)
}
postId = signal(0)
queryClient = injectQueryClient()
result = injectQuery(() => ({
queryKey: ['blogPost', this.postId()],
queryFn: () => fetch(`/blogPosts/${this.postId()}`),
placeholderData: () => {
// Use the smaller/preview version of the blogPost from the 'blogPosts'
// query as the placeholder data for this blogPost query
return queryClient
.getQueryData(['blogPosts'])
?.find((d) => d.id === this.postId())
},
}))
}