How to consume and modify a stream in Workers

function streamToString(source) {
    return new Promise((resolve, reject) => {
        let accumulatedData = '';

        source.onmessage = event => {
            if (event.data === '[DONE]') {
                source.close();
                resolve(accumulatedData);
            } else {
                try {
                    // Directly accumulate data from each message
                    const data = JSON.parse(event.data);
                    accumulatedData += data.response; // Accumulate data into a string
                } catch (error) {
                    reject(error);
                }
            }
        };

        source.onerror = error => {
            reject(error);
        };
    });
}