A simple wrapper for TransformStream.

TransformStreamInterface.ts

/**
 * A simple wrapper for `TransformStream`.
 * @example
 * class UpperCaseStream extends TransformStreamInterface<string, string> {
 *     transform(chunk: string) {
 *         return chunk.toUpperCase()
 *     }
 * }
 *
 * // Usage:
 * const response = await fetch('./lorem-ipsum.txt')
 * if (!response.ok || !response.body) throw new Error('Request failed')
 * const stream = response.body
 *     .pipeThrough(new TextDecoderStream())
 *     .pipeThrough(new UpperCaseStream())
 * // Baseline Newly Available (Safari 26.4+):
 * let fullText = ''
 * for await (const chunk of stream) {
 *     fullText += chunk
 * }
 * console.log(fullText)
 */
abstract class TransformStreamInterface<I, O> extends TransformStream<I, O> {

    constructor() {
        super({
            transform: (chunk: I, controller: TransformStreamDefaultController<O>) =>
                this.internalTransform(chunk, controller)
        })
    }

    private async internalTransform(chunk: I, controller: TransformStreamDefaultController<O>) {
        try {
            const transformedChunk = await this.transform(chunk)
            if (transformedChunk === null) { return }
  
            controller.enqueue(transformedChunk)
        } catch (error) {
            controller.error(error)
        }
    }

    /** Transform the chunk. Return `null` to skip it. */
    protected abstract transform(chunk: I): (O | null) | Promise<O | null>

}