sse-stream-transform/sse-stream-transform.js

SSE Transform Stream for parsing Server-Sent Events (SSE) using Web Streams API

/**
 * SSE (Server-Sent Events) stream transformer.
 *
 * SSE Message format:
 * ```
 * event: <event-name>
 * data: <data-payload>
 * data: <data-payload-line-2>
 * ...
 * ```
 *
 * Each message is separated by a double newline (`\n\n`).
 *
 * The only mandatory field is `data`. Multiple `data` lines are concatenated. The transformer supports concatenation
 * using new lines if specified in options.
 */
/**
 * SSE Stream Transform
 *
 * @example
 * ```ts
 * const sseTransformer = new SseStreamTransform();
 * const response = await fetch("your-sse-endpoint");
 * const readableStream = response.body?.pipeThrough(sseTransformer).pipeThrough(new YourNextTransformStream());
 * ```
 */
export class SseStreamTransform extends TransformStream {
    constructor(options = {}) {
        super({
            async transform(chunk, controller) {
                chunk = await chunk;
                this.buffer += this.decoder.decode(chunk);
                while (this.buffer.includes("\n\n")) {
                    // take the part of the message out of the buffer
                    const index = this.buffer.indexOf("\n\n") + 2;
                    const part = this.buffer.slice(0, index).trim();
                    this.buffer = this.buffer.slice(index);
                    // process message (line by line)
                    const message = part.split("\n").reduce((acc, cur) => {
                        const i = cur.indexOf(":");
                        if (i === -1) {
                            return acc;
                        }
                        const key = cur.slice(0, i).trim();
                        const value = cur.slice(i + 1).trim();
                        if (key === "data" && key in acc) {
                            acc[key] += (options.addNewLines ? "\n" : "") + value;
                        }
                        else {
                            acc[key] = value;
                        }
                        return acc;
                    }, {});
                    if (Object.keys(message).length === 0) {
                        // do not yield empty messages
                        continue;
                    }
                    controller.enqueue(message);
                }
            },
            buffer: "",
            decoder: new TextDecoder(),
        });
    }
}
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