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Bun implements many web standard APIs, making it easier to write code that works across browsers, Node.js, and Bun. These APIs follow the specifications defined by WHATWG and W3C.

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Fetch API

Bun implements the complete Fetch API for making HTTP requests: 
const response = await fetch("https://api.example.com/data");
const data = await response.json();

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fetch()

Global function for HTTP requests: 
fetch(url: string | URL | Request, options?: RequestInit): Promise<Response>

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Request

HTTP request representation: 
const req = new Request("https://api.example.com", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ key: "value" })
});

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Response

HTTP response representation: 
const res = new Response("Hello World", {
  status: 200,
  headers: { "Content-Type": "text/plain" }
});

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Headers

HTTP headers management: 
const headers = new Headers();
headers.set("Content-Type", "application/json");
headers.append("X-Custom-Header", "value");
See fetch documentation for more details.

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Streams API

Bun implements the Web Streams specification:

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ReadableStream

const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("chunk 1");
    controller.enqueue("chunk 2");
    controller.close();
  }
});

for await (const chunk of stream) {
  console.log(chunk);
}

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WritableStream

const writable = new WritableStream({
  write(chunk) {
    console.log("Writing:", chunk);
  }
});

const writer = writable.getWriter();
await writer.write("data");
await writer.close();

​
TransformStream

const transformer = new TransformStream({
  transform(chunk, controller) {
    controller.enqueue(chunk.toUpperCase());
  }
});
See Streams API documentation for complete reference.

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URL & URLSearchParams

Parse and manipulate URLs: 
const url = new URL("https://example.com/path?key=value#hash");
console.log(url.hostname);    // "example.com"
console.log(url.pathname);    // "/path"
console.log(url.searchParams.get("key")); // "value"

const params = new URLSearchParams("key1=value1&key2=value2");
params.append("key3", "value3");
console.log(params.toString()); // "key1=value1&key2=value2&key3=value3"

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FormData

Handle form data: 
const form = new FormData();
form.append("name", "John");
form.append("file", Bun.file("image.png"));

// Send with fetch
await fetch("https://api.example.com/upload", {
  method: "POST",
  body: form
});

​
Blob

Binary data handling: 
const blob = new Blob(["Hello ", "World"], { type: "text/plain" });
const text = await blob.text();
const buffer = await blob.arrayBuffer();
Bun extends Blob with additional methods: 
blob.size          // Size in bytes
blob.type          // MIME type
blob.text()        // Read as text
blob.json()        // Parse as JSON
blob.arrayBuffer() // Read as ArrayBuffer
blob.stream()      // Get ReadableStream
blob.slice()       // Extract portion

​
WebSocket

Real-time communication: 
const ws = new WebSocket("wss://echo.websocket.org");

ws.addEventListener("open", () => {
  ws.send("Hello");
});

ws.addEventListener("message", (event) => {
  console.log("Received:", event.data);
});
For server-side WebSocket, use Bun.serve().

​
Web Crypto API

Cryptographic operations: 
// Generate random values
const array = new Uint8Array(16);
crypto.getRandomValues(array);

// Generate UUID
const uuid = crypto.randomUUID();

// Hashing
const data = new TextEncoder().encode("Hello");
const hashBuffer = await crypto.subtle.digest("SHA-256", data);

​
TextEncoder / TextDecoder

Convert between strings and bytes: 
const encoder = new TextEncoder();
const bytes = encoder.encode("Hello 世界");

const decoder = new TextDecoder("utf-8");
const text = decoder.decode(bytes);

​
Performance

Web performance APIs: 
// High-resolution time
performance.now(); // Microsecond precision

// Measure operations
const start = performance.now();
await someOperation();
const duration = performance.now() - start;
console.log(`Took ${duration}ms`);

​
Timers

Standard timer functions: 
setTimeout(() => console.log("Delayed"), 1000);
setInterval(() => console.log("Repeating"), 1000);
setImmediate(() => console.log("Immediate"));

// Clear timers
const id = setTimeout(() => {}, 1000);
clearTimeout(id);

​
Console

Standard console methods: 
console.log("Message");
console.error("Error");
console.warn("Warning");
console.table([{ a: 1, b: 2 }]);
console.time("operation");
// ... code ...
console.timeEnd("operation");
See Console API documentation for all methods.

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Event APIs

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EventTarget

const target = new EventTarget();

target.addEventListener("custom", (event) => {
  console.log("Event data:", event.detail);
});

target.dispatchEvent(new CustomEvent("custom", {
  detail: { key: "value" }
}));

​
AbortController / AbortSignal

Cancel operations: 
const controller = new AbortController();
const signal = controller.signal;

fetch("https://api.example.com", { signal })
  .catch((err) => {
    if (err.name === "AbortError") {
      console.log("Request cancelled");
    }
  });

// Cancel after 5 seconds
setTimeout(() => controller.abort(), 5000);

​
Additional Web APIs

Bun implements many other web standards:
  • atob() / btoa(): Base64 encoding/decoding
  • structuredClone(): Deep copy objects
  • queueMicrotask(): Schedule microtask
  • MessageChannel / MessagePort: Message passing
  • BroadcastChannel: Cross-context messaging

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Browser Compatibility

Bun’s web APIs are designed to be compatible with browser implementations. Code using these APIs can often run unchanged in browsers. However, some APIs have server-specific behavior:
  • fetch() supports file:// URLs and Unix domain sockets
  • WebSocket has server-side implementation in Bun.serve()
  • Performance APIs may have different precision

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Next Steps

fetch()

Learn about HTTP requests with fetch

Streams

Work with Web Streams API

WebSocket

Real-time communication

Bun APIs

Explore Bun-specific APIs

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