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The deno_web extension provides foundational web platform APIs that other extensions build upon. It implements standards like Streams, TextEncoder/TextDecoder, Blob, File, and more.

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Location

ext/web/

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What It Provides

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Text Encoding

  • TextEncoder - Encodes strings to UTF-8 bytes
  • TextDecoder - Decodes bytes to strings with various encodings
  • Base64 encoding and decoding (atob/btoa)
// Encode text to UTF-8
const encoder = new TextEncoder();
const bytes = encoder.encode("Hello, Deno!");

// Decode bytes to text
const decoder = new TextDecoder();
const text = decoder.decode(bytes);

// Base64 encoding
const encoded = btoa("Hello");
const decoded = atob(encoded);

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Streams API

Implements the WHATWG Streams standard:
  • ReadableStream - For reading data
  • WritableStream - For writing data
  • TransformStream - For transforming data
const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("chunk 1");
    controller.enqueue("chunk 2");
    controller.close();
  }
});

const reader = stream.getReader();
while (true) {
  const { done, value } = await reader.read();
  if (done) break;
  console.log(value);
}

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Blob and File

Binary data handling: 
// Create a Blob
const blob = new Blob(["Hello, ", "World!"], { type: "text/plain" });

// Read Blob contents
const text = await blob.text();
const arrayBuffer = await blob.arrayBuffer();

// Create object URLs
const url = URL.createObjectURL(blob);
URL.revokeObjectURL(url);

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Events

  • Event - Base event class
  • CustomEvent - Custom events with detail
  • EventTarget - Event dispatching
  • AbortSignal / AbortController - Cancellation
const controller = new AbortController();
const signal = controller.signal;

signal.addEventListener("abort", () => {
  console.log("Operation aborted");
});

controller.abort();

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Timers

Standard timer functions:
  • setTimeout / clearTimeout
  • setInterval / clearInterval
  • queueMicrotask
  • performance.now() - High-resolution timestamps
const start = performance.now();

setTimeout(() => {
  const elapsed = performance.now() - start;
  console.log(`Elapsed: ${elapsed}ms`);
}, 100);

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Compression

Compression and decompression streams:
  • CompressionStream - gzip, deflate
  • DecompressionStream - gzip, deflate
const stream = new ReadableStream(/* ... */);
const compressed = stream.pipeThrough(
  new CompressionStream("gzip")
);

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URL Processing

  • URL parsing and manipulation
  • URLSearchParams
  • URLPattern (for URL pattern matching)
const url = new URL("https://deno.land/x/mod.ts?v=1");
console.log(url.hostname); // "deno.land"
console.log(url.searchParams.get("v")); // "1"

// URL pattern matching
const pattern = new URLPattern({ pathname: "/api/:id" });
const match = pattern.exec("https://example.com/api/123");
console.log(match.pathname.groups.id); // "123"

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Message Ports

Cross-context communication: 
const channel = new MessageChannel();

channel.port1.onmessage = (event) => {
  console.log("Received:", event.data);
};

channel.port2.postMessage("Hello!");

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Key Operations

The extension exposes these Rust operations to JavaScript:

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Encoding Operations

op_base64_decode      // Decode base64 strings
op_base64_encode      // Encode to base64
op_encoding_decode    // Decode text with various encodings
op_encoding_encode_into // Encode text to bytes

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Blob Operations

op_blob_create_part         // Create blob part
op_blob_read_part           // Read blob data
op_blob_create_object_url   // Create blob: URL
op_blob_revoke_object_url   // Revoke blob: URL

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Stream Operations

op_readable_stream_resource_allocate  // Create readable stream
op_readable_stream_resource_write_buf // Write to stream
op_readable_stream_resource_close     // Close stream

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Implementation Details

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Extension Definition

deno_core::extension!(deno_web,
  deps = [ deno_webidl ],
  ops = [
    op_base64_decode,
    op_base64_encode,
    op_encoding_decode,
    op_blob_create_part,
    // ... many more
  ],
  esm = [
    "00_infra.js",
    "02_event.js",
    "02_structured_clone.js",
    "06_streams.js",
    "08_text_encoding.js",
    "09_file.js",
    // ...
  ],
  options = {
    blob_store: Arc<BlobStore>,
    maybe_location: Option<Url>,
  }
);

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Blob Storage

Blobs are stored in a shared BlobStore for efficient memory management: 
pub struct BlobStore {
  // Maps blob IDs to their data
  parts: HashMap<Uuid, Vec<u8>>,
}

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Performance Optimizations

  • Fast paths for UTF-8 encoding/decoding
  • SIMD-accelerated base64 operations
  • Zero-copy buffer handling where possible
  • Efficient streaming with backpressure

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Dependencies

  • deno_webidl - WebIDL type conversions
  • External crates:
    • encoding_rs - Character encoding
    • base64-simd - Fast base64
    • url - URL parsing

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File Structure

ext/web/
├── lib.rs              # Extension definition and ops
├── blob.rs             # Blob implementation
├── compression.rs      # Compression streams
├── message_port.rs     # MessagePort/Channel
├── stream_resource.rs  # Stream resources
├── timers.rs          # Timer operations
├── url.rs             # URL operations
├── urlpattern.rs      # URLPattern
├── 00_infra.js        # Core infrastructure
├── 02_event.js        # Event system
├── 06_streams.js      # Streams API
├── 08_text_encoding.js # TextEncoder/Decoder
└── ...

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Testing

The extension is tested through:
  • Unit tests in Rust code
  • Integration tests in cli/tests/
  • Web Platform Tests (WPT) in tests/wpt/

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Standards Compliance

Implements these web standards:

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See Also

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