Splitting Text with TransformStream

I wanted to get better at using the Stream API, so I tried experimenting with string manipulation as an example. This implements a TransformStream that splits long text into arrays of a specific size.

Implementation

type Ctrl = TransformStreamDefaultController<string[]>;

class TextArrayTransformStream extends TransformStream<string, string[]> {
  #chunk: string[] = [];
  #chunkSize: number;
  #splitReg: RegExp;

  constructor(chunkSize: number, maxTextLength: number) {
    super({
      transform: (chunk, controller) => this.#handle(chunk, controller),
      flush: (controller) => this.#flush(controller),
    });
    this.#chunkSize = chunkSize;
    this.#splitReg = new RegExp(`.{1,${maxTextLength}}`, "g");
  }

  #handle(chunk: string, controller: Ctrl): void {
    for (const str of chunk.match(this.#splitReg) || []) {
      if (this.#chunk.length >= this.#chunkSize) {
        controller.enqueue(this.#chunk);
        this.#chunk = [];
      } else {
        this.#chunk.push(str);
      }
    }
  }

  #flush(controller: Ctrl): void {
    if (this.#chunk.length > 0) {
      controller.enqueue(this.#chunk);
    }
  }
}

Helper Function

function toReadableStream(text: string): ReadableStream<string> {
  return new ReadableStream({
    start(controller) {
      controller.enqueue(text);
      controller.close();
    }
  });
}

Usage Example

async function main() {
  const text = "Long text...";
  const arrayLength = 5;    // Group 5 items per array
  const textLength = 10;    // Each element is 10 characters

  const stream = toReadableStream(text)
    .pipeThrough(new TextArrayTransformStream(arrayLength, textLength));

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

    console.log(value); // string[] is output sequentially
  }
}

Use Cases

This pattern is effective in the following scenarios:

When batch processing LLM API responses
When paginating large text displays
Pre-processing before sending to character-limited APIs

Note that AsyncIterator is not implemented, so the for await…of syntax cannot be used.