Streaming with ReadableStream and WritableStream in Deno - Performance & Optimization

Performance: Streaming with ReadableStream and WritableStream

HIGH IMPACT

Streaming affects how quickly data starts to appear on the page and how smoothly it flows, improving load speed and responsiveness.

Loading large data from a server to display progressively

Deno

const response = await fetch(url);
const reader = response.body.getReader();
while(true) {
  const { done, value } = await reader.read();
  if(done) break;
  processChunk(value);
}

Processes data chunks as they arrive, allowing progressive rendering and faster user feedback.

📈 Performance GainReduces LCP by starting rendering early, avoids blocking main thread.

Loading large data from a server to display progressively

Deno

const response = await fetch(url);
const data = await response.text();
processData(data);

Waits for the entire response before processing, blocking rendering and increasing load time.

📉 Performance CostBlocks rendering until full data is received, increasing LCP by hundreds of milliseconds.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Full data fetch before processing	Single large DOM update	1 large reflow	High paint cost due to big update	[X] Bad
Streaming with ReadableStream	Multiple small DOM updates	Multiple small reflows	Lower paint cost per update	[OK] Good

Rendering Pipeline

Streaming data flows through network fetch, then chunk-by-chunk processing triggers incremental layout and paint updates.

→Network

→JavaScript Execution

→Layout

→Paint

⚠️ BottleneckJavaScript Execution when processing large data synchronously

Core Web Vital Affected

LCP

Streaming affects how quickly data starts to appear on the page and how smoothly it flows, improving load speed and responsiveness.

Optimization Tips

1Process data incrementally with ReadableStream to avoid blocking rendering.

2Start rendering content as soon as the first data chunk arrives to improve LCP.

3Avoid waiting for full data before updating the UI to keep the page responsive.

Performance Quiz - 3 Questions

What is the main performance benefit of using ReadableStream for data loading?

A It caches all data before processing to speed up rendering.

B It allows processing data chunks as they arrive, reducing load blocking.

C It compresses data to reduce network size.

D It delays rendering until all data is received.

DevTools: Performance

How to check: Record a performance profile while loading the page. Look for long tasks blocking the main thread and when content first appears.

What to look for: Early content paint marks and shorter long tasks indicate good streaming performance.

Practice

1. What is the main purpose of a ReadableStream in Deno?

easy

A. To accept data chunks that can be written

B. To produce data chunks that can be read

C. To store data permanently

D. To convert data into JSON format

Solution

1. Step 1: Understand the role of ReadableStream

   A ReadableStream is designed to produce data chunks that can be read by consumers.

2. Step 2: Differentiate from WritableStream

   A WritableStream accepts data chunks to be written, which is the opposite role.

3. Final Answer:

   To produce data chunks that can be read -> Option B

4. Quick Check:

   ReadableStream produces data = A [OK]

Hint: ReadableStream produces data; WritableStream consumes data [OK]

Common Mistakes:

• Confusing ReadableStream with WritableStream
• Thinking streams store data permanently
• Assuming streams convert data formats automatically

2. Which of the following is the correct way to create a new WritableStream in Deno?

easy

A. const stream = new WritableStream({ write(chunk) { console.log(chunk); } });

B. const stream = WritableStream.write(chunk => console.log(chunk));

C. const stream = new WritableStream.write(chunk => console.log(chunk));

D. const stream = WritableStream({ write(chunk) { console.log(chunk); } });

Solution

1. Step 1: Recall WritableStream constructor syntax

   WritableStream is created with new WritableStream() and an object with a write method.

2. Step 2: Check each option's syntax

   const stream = new WritableStream({ write(chunk) { console.log(chunk); } }); correctly uses new WritableStream({ write(chunk) { ... } }). Others misuse syntax or omit new.

3. Final Answer:

   const stream = new WritableStream({ write(chunk) { console.log(chunk); } }); -> Option A

4. Quick Check:

   WritableStream needs new + write method object = C [OK]

Hint: Use 'new WritableStream' with an object having a write method [OK]

Common Mistakes:

• Omitting 'new' keyword
• Calling WritableStream as a function
• Passing a function directly instead of an object

3. What will the following code output in Deno?

const readable = new ReadableStream({
  start(controller) {
    controller.enqueue('Hello');
    controller.enqueue('World');
    controller.close();
  }
});

const reader = readable.getReader();

(async () => {
  let result = '';
  while(true) {
    const { done, value } = await reader.read();
    if (done) break;
    result += value + ' ';
  }
  console.log(result.trim());
})();

medium

A. Error: reader.read is not a function

B. HelloWorld

C. Hello World

D. undefined

Solution

1. Step 1: Understand the ReadableStream setup

   The stream enqueues two chunks: 'Hello' and 'World', then closes.

2. Step 2: Analyze the reading loop

   The loop reads each chunk, appends it with a space to result, and stops when done is true.

3. Step 3: Output after trimming

   After reading both chunks, result is 'Hello World ' which trims to 'Hello World'.

4. Final Answer:

   Hello World -> Option C

5. Quick Check:

   Chunks read and concatenated with space = D [OK]

Hint: ReadableStream chunks read sequentially, concatenated with spaces [OK]

Common Mistakes:

• Forgetting to await reader.read()
• Not trimming the final string
• Assuming chunks concatenate without spaces

4. Identify the error in this Deno code using streams:

const writable = new WritableStream({
  write(chunk) {
    console.log(chunk.toUpperCase());
  }
});

const writer = writable.getWriter();

await writer.write(123);
await writer.close();

medium

A. write method must be async

B. Missing await before new WritableStream

C. getWriter() is not a function

D. Cannot call toUpperCase on a number

Solution

1. Step 1: Check the write method usage

   The write method calls chunk.toUpperCase(), which only works on strings.

2. Step 2: Analyze the write call argument

   The code writes the number 123, which has no toUpperCase() method, causing a runtime error.

3. Final Answer:

   Cannot call toUpperCase on a number -> Option D

4. Quick Check:

   toUpperCase on number causes error = A [OK]

Hint: Check data types before calling string methods [OK]

Common Mistakes:

• Assuming write must be async
• Thinking getWriter() is invalid
• Missing await on WritableStream creation

5. You want to transform a ReadableStream of text chunks to uppercase and write them to a WritableStream. Which approach correctly implements this in Deno?

hard

A. Pipe the readable through a TransformStream that uppercases chunks, then pipe to writable

B. Directly write chunks from readable to writable without transformation

C. Use readable.getReader() and writable.getWriter(), but forget to release locks

D. Convert chunks to uppercase after writing to writable

Solution

1. Step 1: Understand stream transformation

   To modify data between readable and writable, use a TransformStream that changes chunks to uppercase.

2. Step 2: Use pipeThrough and pipeTo methods

   Pipe the readable stream through the transform, then pipe the result to the writable stream for correct flow and backpressure handling.

3. Step 3: Evaluate other options

   Directly write chunks from readable to writable without transformation skips transformation, C risks locking issues, and D transforms too late after writing.

4. Final Answer:

   Pipe the readable through a TransformStream that uppercases chunks, then pipe to writable -> Option A

5. Quick Check:

   Use TransformStream + pipeThrough + pipeTo = B [OK]

Hint: Use TransformStream between readable and writable for data changes [OK]

Common Mistakes:

• Skipping TransformStream for data modification
• Not handling stream locks properly
• Transforming data after writing