utils/FileBlob.ts

import { createReadStream } from "node:fs";
import { open, stat } from "node:fs/promises";
import { Readable } from "node:stream";
import type { FileHandle } from "node:fs/promises";
import { fileURLToPath } from "node:url";

/**
 * @internal
 *
 * A FileBlob is a replacement for the Blob class that allows to lazy read files
 * in order to preserve memory.
 *
 * It is a drop-in replacement for the Blob class, so you can use it as a Blob.
 *
 * The main difference is the instantiation, which is done asynchronously using the `FileBlob.create` method.
 *
 * @example
 * const fileBlob = await FileBlob.create("path/to/package.json");
 *
 * await fetch("https://aschen.tech", { method: "POST", body: fileBlob });
 */
export class FileBlob extends Blob {
	/**
	 * Creates a new FileBlob on the provided file.
	 *
	 * @param path Path to the file to be lazy readed
	 */
	static async create(path: string | URL): Promise<FileBlob> {
		path = path instanceof URL ? fileURLToPath(path) : path;

		const { size } = await stat(path);

		const fileBlob = new FileBlob(path, 0, size);

		return fileBlob;
	}

	private path: string;
	private start: number;
	private end: number;

	private constructor(path: string, start: number, end: number) {
		super();

		this.path = path;
		this.start = start;
		this.end = end;
	}

	/**
	 * Returns the size of the blob.
	 */
	override get size(): number {
		return this.end - this.start;
	}

	/**
	 * Returns a new instance of FileBlob that is a slice of the current one.
	 *
	 * The slice is inclusive of the start and exclusive of the end.
	 *
	 * The slice method does not supports negative start/end.
	 *
	 * @param start beginning of the slice
	 * @param end end of the slice
	 */
	override slice(start = 0, end = this.size): FileBlob {
		if (start < 0 || end < 0) {
			new TypeError("Unsupported negative start/end on FileBlob.slice");
		}

		const slice = new FileBlob(this.path, this.start + start, Math.min(this.start + end, this.end));

		return slice;
	}

	/**
	 * Read the part of the file delimited by the FileBlob and returns it as an ArrayBuffer.
	 */
	override async arrayBuffer(): Promise<ArrayBuffer> {
		const slice = await this.execute((file) => file.read(Buffer.alloc(this.size), 0, this.size, this.start));

		return slice.buffer;
	}

	/**
	 * Read the part of the file delimited by the FileBlob and returns it as a string.
	 */
	override async text(): Promise<string> {
		const buffer = (await this.arrayBuffer()) as Buffer;

		return buffer.toString("utf8");
	}

	/**
	 * Returns a stream around the part of the file delimited by the FileBlob.
	 */
	override stream(): ReturnType<Blob["stream"]> {
		return Readable.toWeb(createReadStream(this.path, { start: this.start, end: this.end - 1 })) as ReturnType<
			Blob["stream"]
		>;
	}

	/**
	 * We are opening and closing the file for each action to prevent file descriptor leaks.
	 *
	 * It is an intended choice of developer experience over performances.
	 */
	private async execute<T>(action: (file: FileHandle) => Promise<T>) {
		const file = await open(this.path, "r");

		try {
			return await action(file);
		} finally {
			await file.close();
		}
	}
}