Streaming tests
Provide predictable delayed output while testing code that consumes streams.
AI Tools · Function
simulateReadableStream
Creates a ReadableStream that emits supplied values in order with optional initial and inter-chunk delays. It is mainly useful for testing streaming behavior or reproducing time-delayed responses.
How it works
The utility builds a generic ReadableStream from an ordered chunks array. It can pause before the first emission and between later emissions, or skip either delay with null, then closes after the final chunk.
Syntax
simulateReadableStream<T>({ chunks, initialDelayInMs?, chunkDelayInMs? }): ReadableStream<T>Parameters
| Name | Type | Description |
|---|---|---|
chunks | T[] | Array of values to emit sequentially. |
initialDelayInMs | number | null | Optional milliseconds to wait before the first value; defaults to 0, or null to skip the initial delay. |
chunkDelayInMs | number | null | Optional milliseconds to wait between values; defaults to 0, or null to skip inter-chunk delays. |
Returns
ReadableStream<T> that emits the provided chunks in sequence and closes after the last one.
Key points
- It is a generic helper for producing ordered stream values while controlling simulated latency.
- The required chunks option supplies the complete sequence the stream will publish.
- initialDelayInMs affects only the pause before the first chunk.
- chunkDelayInMs applies between consecutive chunks, not before the first one.
- Consumers receive a normal completion after the final array element.
Common mistakes
- Use null rather than relying on an omitted option when the first delay must be explicitly disabled.
- A null chunk delay makes subsequent values arrive without an intentional pause.
Example: Simulate delayed text chunks
import { simulateReadableStream } from 'ai';
const stream = simulateReadableStream({
chunks: ['Hello', ' ', 'world!'],
initialDelayInMs: 100,
chunkDelayInMs: 50,
});
for await (const chunk of stream) {
process.stdout.write(chunk);
}Creates a stream that emits three strings after an initial pause and then at fixed intervals.
When to use it
Cached response playback
Recreate progressive model-like output from saved chunks.
Latency simulation
Exercise UI or transport behavior under controlled latency.
Interview questions
QWhat does simulateReadableStream do?
It accepts an array of values and returns a ReadableStream that emits them in order. The stream closes automatically once every value has been emitted.
Follow-up: How would you model a stream of structured events?
QHow do the delay options differ?
initialDelayInMs controls the wait before the first value, while chunkDelayInMs controls the wait between later values. Both default to zero when omitted.
Follow-up: How can you create a stream with no artificial timing?
QHow do you disable delays?
Set either delay option to null to omit that delay entirely. This is useful when tests need deterministic immediate delivery rather than simulated latency.
Follow-up: How could you test a consumer's handling of progressive output?
Related
ReadableStreamstreamTextMockLanguageModelV4