feat(bundlesize): export readSizes from the package entry

packages/bundlesize/README.md

@@ -444,6 +444,16 @@ import { compareSizes } from "@node-cli/bundlesize";
 const { data, passed } = compareSizes({ current, baseline, baseSha });
 ```
 
+So is the reader `--type compare` uses for both files, to load a stored baseline with the same checks:
+
+```js
+import { readSizes } from "@node-cli/bundlesize";
+
+const { sizes, error } = readSizes("tmp/baseline/sizes.json");
+```
+
+It never throws. On success `error` is `""`; otherwise `sizes` is `[]` and `error` says what was wrong: the file could not be read or parsed, it holds the `{ error }` object `size-limit --json` prints when its own run fails, it is not an array of `{ name, size, sizeLimit?, passed? }` with non-negative sizes, or two entries share a `name`.
+
 ## Get help
 
 ```sh

packages/bundlesize/src/__tests__/readSizes.test.ts

@@ -0,0 +1,77 @@
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+// The package's "." export, so this also locks the public surface.
+import { readSizes } from "../config.js";
+
+let dir: string;
+const write = (name: string, content: unknown) => {
+	const file = join(dir, name);
+	writeFileSync(
+		file,
+		typeof content === "string" ? content : JSON.stringify(content),
+	);
+	return file;
+};
+
+beforeEach(() => {
+	dir = mkdtempSync(join(tmpdir(), "bundlesize-read-sizes-"));
+});
+
+afterEach(() => {
+	rmSync(dir, { recursive: true, force: true });
+});
+
+describe("readSizes", () => {
+	it("is exported from the package entry", () => {
+		expect(typeof readSizes).toBe("function");
+	});
+
+	it("returns the entries of a valid sizes file", () => {
+		const sizes = [
+			{ name: "first-paint", passed: true, size: 2048, sizeLimit: 4096 },
+			{ name: "lazy", size: 0 },
+		];
+		expect(readSizes(write("sizes.json", sizes))).toEqual({
+			sizes,
+			error: "",
+		});
+	});
+
+	it("reports a file that cannot be read", () => {
+		const result = readSizes(join(dir, "missing.json"));
+		expect(result.sizes).toEqual([]);
+		expect(result.error).toMatch(/^Failed to read JSON file: /);
+	});
+
+	it("surfaces the error size-limit prints when its own run fails", () => {
+		const file = write("sizes.json", { error: "Cannot find dist/main.js" });
+		expect(readSizes(file)).toEqual({
+			sizes: [],
+			error: `Sizes file ${file} reports an error: Cannot find dist/main.js`,
+		});
+	});
+
+	it.each([
+		["a non-array", { name: "a", size: 1 }],
+		["a missing name", [{ size: 1 }]],
+		["a negative size", [{ name: "a", size: -1 }]],
+		["a non-numeric limit", [{ name: "a", size: 1, sizeLimit: "1 kB" }]],
+		["a non-boolean passed", [{ name: "a", size: 1, passed: "yes" }]],
+	])("rejects %s", (_label, content) => {
+		const result = readSizes(write("sizes.json", content));
+		expect(result.sizes).toEqual([]);
+		expect(result.error).toMatch(/^Invalid sizes file /);
+	});
+
+	it("rejects duplicate entry names", () => {
+		const file = write("sizes.json", [
+			{ name: "a", size: 1 },
+			{ name: "a", size: 2 },
+		]);
+		const result = readSizes(file);
+		expect(result.sizes).toEqual([]);
+		expect(result.error).toContain('Duplicate entry name "a"');
+	});
+});

packages/bundlesize/src/config.ts

@@ -6,6 +6,7 @@ export {
 	compareSizes,
 	type SizeResult,
 } from "./compare.js";
+export { readSizes } from "./compareStats.js";
 
 /**
  * Configuration for a single file size entry.