Stream an order book

// Show a market's order book for both outcomes, Yes and No, as it changes.
// The API sends the Yes book. The No book holds the same orders seen from the
// other side: a Yes bid at 0.40 is a No ask at 0.60. So the sides swap, and each
// price becomes its complement.
// Runs in browsers and in Node.js 22 or later, which have WebSocket built in.
// Get an API key from the developer console: https://developers.predict.fun
const apiKey = "YOUR_API_KEY";
// a market ID, like the ones the Market data example prints.
const marketId = "MARKET_ID";

type Level = [price: number, shares: number];

// Prices have the market's decimal precision, for example 2 for steps of 0.01.
const response = await fetch(`https://api.predict.fun/v1/markets/${marketId}`, {
  headers: { "x-api-key": apiKey },
});
if (!response.ok) throw new Error(`Predict API error: ${response.status}`);
const { decimalPrecision } = (await response.json()).data;

// The No price of a Yes price. Rounding at the market's precision makes the two
// add up to exactly 1.
function complement(price: number): number {
  const factor = 10 ** decimalPrecision;
  return (factor - Math.round(price * factor)) / factor;
}

function cents(price: number): string {
  return `${(price * 100).toFixed(Math.max(decimalPrecision - 2, 0))}¢`;
}

// Prints the 5 best levels of each side. The asks print highest first, so the
// best ask and the best bid meet at the spread.
function printBook(outcome: string, asks: Level[], bids: Level[]) {
  const row = (side: string, [price, shares]: Level) =>
    `  ${side}  ${cents(price).padStart(6)}  ${shares.toFixed(2).padStart(10)}`;
  console.log(`${outcome}       price      shares`);
  for (const level of asks.slice(0, 5).reverse()) {
    console.log(row("ask", level));
  }
  if (asks.length > 0 && bids.length > 0) {
    console.log(`  spread ${cents(asks[0][0] - bids[0][0])}`);
  }
  for (const level of bids.slice(0, 5)) {
    console.log(row("bid", level));
  }
}

// Browsers can't set headers on a WebSocket, so the key goes in the URL.
const ws = new WebSocket(`wss://ws.predict.fun/ws?apiKey=${apiKey}`);
const topic = `predictOrderbook/${marketId}`;

ws.addEventListener("open", () => {
  ws.send(
    JSON.stringify({ method: "subscribe", requestId: 1, params: [topic] }),
  );
});

ws.addEventListener("message", (event) => {
  const message = JSON.parse(event.data);
  if (message.type === "R" && !message.success) {
    console.error(`Subscribe failed: ${message.error.message}`);
  } else if (message.topic === "heartbeat") {
    // Echo each heartbeat, or the server closes the connection.
    ws.send(JSON.stringify({ method: "heartbeat", data: message.data }));
  } else if (message.topic === topic) {
    const { asks, bids }: { asks: Level[]; bids: Level[] } = message.data;
    const toNo = ([price, shares]: Level): Level => [complement(price), shares];
    console.clear();
    printBook("Yes", asks, bids);
    // No asks come from the Yes bids, and No bids from the Yes asks.
    printBook("No ", bids.map(toNo), asks.map(toNo));
  }
});

ws.addEventListener("close", (event) => {
  console.log(`Connection closed (${event.code})`);
});