fix: check postcss as project dependency

src/index.js

@@ -1,3 +1,5 @@
+import path from "path";
+
 import postcss from "postcss";
 import { satisfies } from "semver";
 import postcssPackage from "postcss/package.json";
@@ -11,8 +13,13 @@ import {
   exec,
   normalizeSourceMap,
   normalizeSourceMapAfterPostcss,
+  parsePackageJson,
+  findPackageJsonDir,
 } from "./utils";
 
+let hasExplicitDependencyOnPostCSS = false;
+let packageJsonDir;
+
 /**
  * **PostCSS Loader**
  *
@@ -26,7 +33,6 @@ import {
  *
  * @return {callback} callback Result
  */
-
 export default async function loader(content, sourceMap, meta) {
   const options = this.getOptions(schema);
   const callback = this.async();
@@ -102,6 +108,32 @@ export default async function loader(content, sourceMap, meta) {
       processOptions
     );
   } catch (error) {
+    // The `findPackageJsonDir` function returns `string` or `null`.
+    // This is used to do for caching, that is, an explicit comparison with `undefined`
+    // is used to make the condition body run once.
+    if (packageJsonDir === undefined) {
+      packageJsonDir = findPackageJsonDir(process.cwd(), this.fs.statSync);
+    }
+    // Check postcss versions to avoid using PostCSS 7.
+    // For caching reasons, we use the readFileSync and existsSync functions from the context,
+    // not the functions from the `fs` module.
+    if (
+      !hasExplicitDependencyOnPostCSS &&
+      postcssFactory().version.startsWith("7.") &&
+      packageJsonDir
+    ) {
+      const filePath = path.resolve(packageJsonDir, "package.json");
+      const pkg = parsePackageJson(filePath, this.fs.readFileSync);
+      if (!pkg.dependencies.postcss && !pkg.devDependencies.postcss) {
+        this.emitWarning(
+          "Add postcss as project dependency. postcss is not a peer dependency for postcss-loader. " +
+            "Use `npm install postcss` or `yarn add postcss`"
+        );
+      } else {
+        hasExplicitDependencyOnPostCSS = true;
+      }
+    }
+
     if (error.file) {
       this.addDependency(error.file);
     }

src/utils.js

@@ -408,10 +408,33 @@ function normalizeSourceMapAfterPostcss(map, resourceContext) {
   return newMap;
 }
 
+function parsePackageJson(filePath, readFileSync) {
+  return JSON.parse(readFileSync(filePath, "utf8"));
+}
+
+function findPackageJsonDir(cwd, statSync) {
+  let dir = cwd;
+  for (;;) {
+    try {
+      if (statSync(path.join(dir, "package.json")).isFile()) break;
+      // eslint-disable-next-line no-empty
+    } catch (error) {}
+    const parent = path.dirname(dir);
+    if (dir === parent) {
+      dir = null;
+      break;
+    }
+    dir = parent;
+  }
+  return dir;
+}
+
 export {
   loadConfig,
   getPostcssOptions,
   exec,
   normalizeSourceMap,
   normalizeSourceMapAfterPostcss,
+  parsePackageJson,
+  findPackageJsonDir,
 };

test/__snapshots__/loader.test.js.snap

@@ -1,5 +1,12 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`check postcss versions to avoid using PostCSS 7 should emit a warning if postcss version is not explicitly specified when the loader is failed: warnings 1`] = `
+Array [
+  "ModuleWarning: Module Warning (from \`replaced original path\`):
+(Emitted value instead of an instance of Error) Add postcss as project dependency. postcss is not a peer dependency for postcss-loader. Use \`npm install postcss\` or \`yarn add postcss\`",
+]
+`;
+
 exports[`loader should emit asset using the "messages" API: errors 1`] = `Array []`;
 
 exports[`loader should emit asset using the "messages" API: warnings 1`] = `Array []`;

test/loader.test.js

@@ -2,6 +2,9 @@ import path from "path";
 
 import postcss from "postcss";
 
+// eslint-disable-next-line import/no-namespace
+import * as utils from "../src/utils";
+
 import {
   compile,
   getCompiler,
@@ -198,3 +201,45 @@ describe("loader", () => {
     expect(getErrors(stats)).toMatchSnapshot("errors");
   });
 });
+
+describe("check postcss versions to avoid using PostCSS 7", async () => {
+  async function getStats() {
+    const compiler = getCompiler("./css/index.js", {
+      implementation: (...args) => {
+        const result = postcss(...args);
+        result.version = "7.0.0";
+        result.process = () =>
+          Promise.reject(new Error("Something went wrong."));
+        return result;
+      },
+    });
+    return compile(compiler);
+  }
+
+  it("should emit a warning if postcss version is not explicitly specified when the loader is failed", async () => {
+    jest
+      .spyOn(utils, "parsePackageJson")
+      .mockReturnValue({ dependencies: {}, devDependencies: {} });
+    const stats = await getStats();
+    expect(getWarnings(stats)).toMatchSnapshot("warnings");
+  });
+
+  it("should not show a warning if postcss version is explicitly defined", async () => {
+    jest.spyOn(utils, "parsePackageJson").mockReturnValue({
+      dependencies: {},
+      devDependencies: { postcss: "8.0.0" },
+    });
+    const stats = await getStats();
+    expect(stats.compilation.warnings.length).toBe(0);
+  });
+
+  it("should not show a warning if the package.json file was not found", async () => {
+    jest.spyOn(utils, "findPackageJsonDir").mockReturnValue(null);
+    jest.spyOn(utils, "parsePackageJson").mockReturnValue({
+      dependencies: {},
+      devDependencies: { postcss: "8.0.0" },
+    });
+    const stats = await getStats();
+    expect(stats.compilation.warnings.length).toBe(0);
+  });
+});