batchjs/dist/esm/streams/classes/FirstStream.js

Batch processing framework for NodeJS

import { DiscardingSingleObjectDuplex } from "../interfaces/_index";
/**
 * @class
 * Class that allows you to emit only the first chunk in a stream and discard the rest.
 * @extends DiscardingSingleObjectDuplex
 * @template T
 * @example
 * ```typescript
 * const stream:FirstStream<string> = new FirstStream({
 *     objectMode: true,
 *     matcher: (chunk: string) => chunk.length > 2
 * });
 *
 * stream.write("first");
 * stream.write("second");// Discarded
 * stream.write("third");// Discarded
 * stream.end();
 *
 * stream.on("data", (chunk: string) => {
 *     console.log(``Pushed chunk: ${chunk}```);
 * });
 * stream.on("discard", (chunk: string) => {
 *     console.log(``Discarded chunk: ${chunk}```);
 * });
 * ```
 * ```shell
 * >> Pushed chunk: first
 * >> Discarded chunk: second
 * >> Discarded chunk: third
 * ```
 */
export class FirstStream extends DiscardingSingleObjectDuplex {
    /**
     * @constructor
     * @param {ObjectDuplexOptions} options - The options for the FirstStream.
     */
    constructor(options) {
        super(options, () => true);
    }
    /**
     * A method to write data to the stream, save first chunk and discard the rest, and execute the callback.
     *
     * @param {T} chunk - The data chunk to write to the stream.
     * @param {BufferEncoding} encoding - The encoding of the data.
     * @param {TransformCallback} callback - The callback function to be executed after writing the data.
     * @return {void} This function does not return anything.
     */
    _write(chunk, encoding, callback) {
        if (this.result === undefined) {
            this.result = chunk;
            this._flush();
        }
        else {
            this.emit("discard", chunk);
        }
        callback();
    }
}