Streaming with ReadableStream and WritableStream in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a ReadableStream that outputs the string 'Hello'.

Deno

const stream = new ReadableStream({
  start(controller) {
    controller.[1]('Hello');
    controller.close();
  }
});

Drag options to blanks, or click blank then click option'

Asend

Benqueue

Cpush

Dwrite

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'write' which is for WritableStream, not ReadableStream.

Using 'push' or 'send' which are not valid methods here.

The enqueue method adds chunks to the stream's internal queue.

2fill in blank

medium

Complete the code to create a WritableStream that logs each chunk it receives.

Deno

const writable = new WritableStream({
  write(chunk) {
    console.[1](chunk);
  }
});

Drag options to blanks, or click blank then click option'

Alog

Berror

Cwarn

Dinfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'error' or 'warn' which are for warnings or errors, not general logs.

Using 'info' which may not be supported in all environments.

The console.log method outputs messages to the console, suitable for logging chunks.

3fill in blank

hard

Fix the error in the code to pipe a ReadableStream to a WritableStream.

Deno

await readable.[1](writable);

Drag options to blanks, or click blank then click option'

ApipeTo

BconnectTo

CpipeFrom

DsendTo

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pipeFrom' which does not exist on ReadableStream.

Using 'connectTo' or 'sendTo' which are not valid methods.

The pipeTo method connects a ReadableStream to a WritableStream, forwarding data.

4fill in blank

hard

Fill both blanks to create a ReadableStream that outputs numbers 1 to 3 and then closes.

Deno

const stream = new ReadableStream({
  start(controller) {
    for(let i = 1; i <= 3; i++) {
      controller.[1](i);
    }
    controller.[2]();
  }
});

Drag options to blanks, or click blank then click option'

Aenqueue

Bwrite

Cclose

Dend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'write' which is for WritableStream, not ReadableStream.

Using 'end' which is not a method on ReadableStream controller.

enqueue adds chunks to the stream, and close signals the end of the stream.

5fill in blank

hard

Fill all three blanks to create a WritableStream that converts chunks to uppercase and logs them.

Deno

const writable = new WritableStream({
  write([1]) {
    const upper = [2].toUpperCase();
    console.[3](upper);
  }
});

Drag options to blanks, or click blank then click option'

Achunk

Clog

Dwrite

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and variable.

Using 'write' instead of 'log' for console output.

The parameter is named chunk, which is converted to uppercase and logged with console.log.

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