Streaming with ReadableStream and WritableStream in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a ReadableStream in Deno?

A ReadableStream is a source of data you can read from. It lets you get data piece by piece instead of all at once, like reading a book page by page instead of the whole book at once.

beginner

What does a WritableStream do?

A WritableStream is a destination where you can send data to be written or saved. Think of it like pouring water into a bottle; the stream accepts data chunks and stores or processes them.

intermediate

How do you connect a ReadableStream to a WritableStream in Deno?

You use the pipeTo() method on the ReadableStream to send its data directly to a WritableStream. This is like connecting a hose from a faucet (readable) to a bucket (writable).

beginner

What is the benefit of streaming data instead of reading or writing it all at once?

Streaming saves memory and time by handling data in small parts. It’s like eating a sandwich bite by bite instead of trying to swallow it whole. This is useful for large files or slow networks.

intermediate

What role does the reader and writer play in streams?

A <code>reader</code> reads chunks from a <code>ReadableStream</code>, and a <code>writer</code> writes chunks to a <code>WritableStream</code>. They let you control how data flows step-by-step.

Which method connects a ReadableStream to a WritableStream in Deno?

A pipeTo()

B connect()

C write()

D read()

The pipeTo() method sends data from a ReadableStream directly to a WritableStream.

What does a WritableStream do?

A Writes data to a destination

B Creates a new file

C Reads data from a source

D Deletes data

A WritableStream accepts data chunks to be written or saved.

Why is streaming data useful?

A It loads all data at once

B It saves memory by processing data in chunks

C It slows down data transfer

D It deletes data after reading

Streaming processes data in small parts, saving memory and improving efficiency.

In Deno, what do you use to read chunks from a ReadableStream?

A writer

B streamer

C pipeTo

D reader

A reader reads chunks from a ReadableStream.

What happens if you call pipeTo() on a ReadableStream?

A It closes the stream immediately

B It reads data from a WritableStream

C It sends data to a WritableStream

D It creates a new ReadableStream

pipeTo() sends data from the ReadableStream to the WritableStream.

Explain how you would use ReadableStream and WritableStream together to handle large file data in Deno.

Think about reading and writing data bit by bit instead of all at once.

Describe the roles of reader and writer in the context of streams in Deno.

Imagine a reader as someone reading pages and a writer as someone writing pages.

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