2024-01-19 Updated: 2024-01-19

npm startでエラー 'React' must be in scope when using JSX

背景

Reactの本を買ったので、一年半ぶりにReactの記事を書きます。

書籍に書いてある通りcreate-react-appを実行して作成されたアプリケーションのディレクトリに移動します。

1
2
3

$ npx create-react-app my-react
$ cd my-react
$ npm start

すると、ブラウザに表示された画面は真っ白で、コンソールには

> my-react@0.1.0 start
> react-scripts start
Starting the development server...
Failed to compile.

./src/App.js
  Line 7:5:    'React' must be in scope when using JSX  react/react-in-jsx-scope
  Line 8:7:    'React' must be in scope when using JSX  react/react-in-jsx-scope
  Line 9:9:    'React' must be in scope when using JSX  react/react-in-jsx-scope
  Line 10:9:   'React' must be in scope when using JSX  react/react-in-jsx-scope
  Line 11:16:  'React' must be in scope when using JSX  react/react-in-jsx-scope
  Line 13:9:   'React' must be in scope when using JSX  react/react-in-jsx-scope

Search for the keywords to learn more about each error.

と表示されてしまいました。

原因

調べてみると、src/App.jsの先頭行に以下の記述が必要だったようです。

1	import React from 'react';

一旦この行を追加してから実行してみます。

$ npm start

> my-react@0.1.0 start
> react-scripts start
Starting the development server...
Compiled successfully!

You can now view my-react in the browser.

  Local:            http://localhost:3000/
  On Your Network:  http://192.168.11.7:3000/

Note that the development build is not optimized.
To create a production build, use npm run build.

確かに問題なく動作しました。
であれば、なぜその行の記述がなかったのでしょうか。

JSX Transform

React17から導入されたJSX Transfromでは、先ほど記述したimport文が不要になるというメリットがあるようです。
なにが変わったかというと、トランスパイル後のコードでReactを読み込まなくても動作するようになるということのようです。

では、なぜ、今回その行を追加しないとエラーになってしまったのでしょうか。

ESLint

調べてみるとESLintの設定でエラーを抑制するように書かれていました。
package.jsonを変更しながら、ruleの箇所を追加したり、extendsを変更したりしました。
しかし、結果的に修正できませんでした。

npm install react-scripts@latest

最後の方法として、react-scriptsを最新版にすることでした。

$ npm install react-scripts@latest
npm WARN deprecated @babel/plugin-proposal-private-methods@7.18.6: This proposal has been merged to the ECMAScript standard and thus this plugin is no longer maintained. Please use @babel/plugin-transform-private-methods instead.
npm WARN deprecated @babel/plugin-proposal-numeric-separator@7.18.6: This proposal has been merged to the ECMAScript standard and thus this plugin is no longer maintained. Please use @babel/plugin-transform-numeric-separator instead.
npm WARN deprecated @babel/plugin-proposal-nullish-coalescing-operator@7.18.6: This proposal has been merged to the ECMAScript standard and thus this plugin is no longer maintained. Please use @babel/plugin-transform-nullish-coalescing-operator instead.
npm WARN deprecated @babel/plugin-proposal-class-properties@7.18.6: This proposal has been merged to the ECMAScript standard and thus this plugin is no longer maintained. Please use @babel/plugin-transform-class-properties instead.
npm WARN deprecated rollup-plugin-terser@7.0.2: This package has been deprecated and is no longer maintained. Please use @rollup/plugin-terser
npm WARN deprecated @babel/plugin-proposal-optional-chaining@7.21.0: This proposal has been merged to the ECMAScript standard and thus this plugin is no longer maintained. Please use @babel/plugin-transform-optional-chaining instead.
npm WARN deprecated domexception@2.0.1: Use your platform's native DOMException instead
npm WARN deprecated sourcemap-codec@1.4.8: Please use @jridgewell/sourcemap-codec instead
npm WARN deprecated workbox-cacheable-response@6.6.0: workbox-background-sync@6.6.0

added 567 packages, removed 766 packages, changed 382 packages, and audited 1555 packages in 1m

254 packages are looking for funding
  run `npm fund` for details

8 vulnerabilities (2 moderate, 6 high)

To address all issues (including breaking changes), run:
  npm audit fix --force

Run `npm audit` for details.

deprecatedは一旦置いておいて、その後にnpm startします。

$ npm start

> my-react@0.1.0 start
> react-scripts start

(node:79721) [DEP_WEBPACK_DEV_SERVER_ON_AFTER_SETUP_MIDDLEWARE] DeprecationWarning: 'onAfterSetupMiddleware' option is deprecated. Please use the 'setupMiddlewares' option.
(Use `node --trace-deprecation ...` to show where the warning was created)
(node:79721) [DEP_WEBPACK_DEV_SERVER_ON_BEFORE_SETUP_MIDDLEWARE] DeprecationWarning: 'onBeforeSetupMiddleware' option is deprecated. Please use the 'setupMiddlewares' option.
Starting the development server...
Compiled successfully!

You can now view my-react in the browser.

  Local:            http://localhost:3000
  On Your Network:  http://192.168.11.7:3000

Note that the development build is not optimized.
To create a production build, use npm run build.

webpack compiled successfully

無事動作しました。

何が変わったのか

何が変わったのかをみてみると、package.jsonの差分は1行だけでした。

$ git diff package.json
diff --git a/package.json b/package.json
index 28afd0a..bd487ff 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,7 @@
     "@testing-library/user-event": "^13.5.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
-    "react-scripts": "5.0.1",
+    "react-scripts": "^5.0.1",
     "web-vitals": "^2.1.4"
   },
   "scripts": {

package-lock.jsonをみると、結構変わっています。

$ git diff package-lock.json
diff --git a/package-lock.json b/package-lock.json
index ba7b477..f91a96e 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -13,7 +13,7 @@
         "@testing-library/user-event": "^13.5.0",
         "react": "^18.2.0",
         "react-dom": "^18.2.0",
-        "react-scripts": "5.0.1",
+        "react-scripts": "^5.0.1",
         "web-vitals": "^2.1.4"
       }
     },
@@ -257,9 +257,9 @@
       }
     },
     "node_modules/@babel/helper-define-polyfill-provider": {
-      "version": "0.4.4",
-      "resolved": "https://registry.npmjs.org/@babel/helper-define-polyfill-provider/-/helper-define-polyfill-provider-0.4.4.tgz",
-      "integrity": "sha512-QcJMILQCu2jm5TFPGA3lCpJJTeEP+mqeXooG/NZbg/h5FTFi6V0+99ahlRsW8/kRLyb24LZVCCiclDedhLKcBA==",
+      "version": "0.5.0",
+      "resolved": "https://registry.npmjs.org/@babel/helper-define-polyfill-provider/-/helper-define-polyfill-provider-0.5.0.tgz",
+      "integrity": "sha512-NovQquuQLAQ5HuyjCz7WQP9MjRj7dx++yspwiyUiGl9ZyadHRSql1HZh5ogRd8W8w6YM6EQ/NTB8rgjLt5W65Q==",
       "dependencies": {
         "@babel/helper-compilation-targets": "^7.22.6",
         "@babel/helper-plugin-utils": "^7.22.5",
@@ -3427,9 +3427,9 @@
       "integrity": "sha512-EYNwp3bU+98cpU4lAWYYL7Zz+2gryWH1qbdDTidVd6hkiR6weksdbMadyXKXNPEkQFhXM+hVO9ZygomHXp+AIw=="
     },
     "node_modules/@rushstack/eslint-patch": {
-      "version": "1.6.1",
-      "resolved": "https://registry.npmjs.org/@rushstack/eslint-patch/-/eslint-patch-1.6.1.tgz",
-      "integrity": "sha512-UY+FGM/2jjMkzQLn8pxcHGMaVLh9aEitG3zY2CiY7XHdLiz3bZOwa6oDxNqEMv7zZkV+cj5DOdz0cQ1BP5Hjgw=="
+      "version": "1.7.0",
+      "resolved": "https://registry.npmjs.org/@rushstack/eslint-patch/-/eslint-patch-1.7.0.tgz",
+      "integrity": "sha512-Jh4t/593gxs0lJZ/z3NnasKlplXT2f+4y/LZYuaKZW5KAaiVFL/fThhs+17EbUd53jUVJ0QudYCBGbN/psvaqg=="
     },
     "node_modules/@sinclair/typebox": {
       "version": "0.24.51",
@@ -5359,9 +5359,9 @@
       }
     },
     "node_modules/autoprefixer": {
-      "version": "10.4.16",
-      "resolved": "https://registry.npmjs.org/autoprefixer/-/autoprefixer-10.4.16.tgz",
-      "integrity": "sha512-7vd3UC6xKp0HLfua5IjZlcXvGAGy7cBAXTg2lyQ/8WpNhd6SiZ8Be+xm3FyBSY
Jx5GKcpRCzBh7RH4/0dnY+uQ==",
+      "version": "10.4.17",
+      "resolved": "https://registry.npmjs.org/autoprefixer/-/autoprefixer-10.4.17.tgz",
+      "integrity": "sha512-/cpVNRLSfhOtcGflT13P2794gVSgmPgTR+erw5ifnMLZb0UnSlkK4tquLmkd3B
hA+nLo5tX8Cu0upUsGKvKbmg==",
       "funding": [
         {
           "type": "opencollective",
@@ -5377,9 +5377,9 @@
         }
       ],
       "dependencies": {
-        "browserslist": "^4.21.10",
-        "caniuse-lite": "^1.0.30001538",
-        "fraction.js": "^4.3.6",
+        "browserslist": "^4.22.2",
+        "caniuse-lite": "^1.0.30001578",
+        "fraction.js": "^4.3.7",
         "normalize-range": "^0.1.2",
         "picocolors": "^1.0.0",
         "postcss-value-parser": "^4.2.0"
@@ -5593,12 +5593,12 @@
       }
     },
     "node_modules/babel-plugin-polyfill-corejs2": {
-      "version": "0.4.7",
-      "resolved": "https://registry.npmjs.org/babel-plugin-polyfill-corejs2/-/babel-plugin-polyfill-corejs2-0.4.7.tgz",
-      "integrity": "sha512-LidDk/tEGDfuHW2DWh/Hgo4rmnw3cduK6ZkOI1NPFceSK3n/yAGeOsNT7FLnSGHkXj3RHGSEVkN3FsCTY6w2CQ==",
+      "version": "0.4.8",
+      "resolved": "https://registry.npmjs.org/babel-plugin-polyfill-corejs2/-/babel-plugin-polyfill-corejs2-0.4.8.tgz",
+      "integrity": "sha512-OtIuQfafSzpo/LhnJaykc0R/MMnuLSSVjVYy9mHArIZ9qTCSZ6TpWCuEKZYVoN//t8HqBNScHrOtCrIK5IaGLg==",
       "dependencies": {
         "@babel/compat-data": "^7.22.6",
-        "@babel/helper-define-polyfill-provider": "^0.4.4",
+        "@babel/helper-define-polyfill-provider": "^0.5.0",
         "semver": "^6.3.1"
       },
       "peerDependencies": {
@@ -5625,12 +5625,27 @@
         "@babel/core": "^7.4.0 || ^8.0.0-0 <8.0.0"
       }
     },
+    "node_modules/babel-plugin-polyfill-corejs3/node_modules/@babel/helper-define-polyfill-provider": {
+      "version": "0.4.4",
+      "resolved": "https://registry.npmjs.org/@babel/helper-define-polyfill-provider/-/helper-define-polyfill-provider-0.4.4.tgz",
+      "integrity": "sha512-QcJMILQCu2jm5TFPGA3lCpJJTeEP+mqeXooG/NZbg/h5FTFi6V0+99ahlRsW8/kRLyb24LZVCCiclDedhLKcBA==",
+      "dependencies": {
+        "@babel/helper-compilation-targets": "^7.22.6",
+        "@babel/helper-plugin-utils": "^7.22.5",
+        "debug": "^4.1.1",
+        "lodash.debounce": "^4.0.8",
+        "resolve": "^1.14.2"
+      },
+      "peerDependencies": {
+        "@babel/core": "^7.4.0 || ^8.0.0-0 <8.0.0"
+      }
+    },
     "node_modules/babel-plugin-polyfill-regenerator": {
-      "version": "0.5.4",
-      "resolved": "https://registry.npmjs.org/babel-plugin-polyfill-regenerator/-/babel-plugin-polyfill-regenerator-0.5.4.tgz",
-      "integrity": "sha512-S/x2iOCvDaCASLYsOOgWOq4bCfKYVqvO/uxjkaYyZ3rVsVE3CeAI/c84NpyuBBymEgNvHgjEot3a9/Z/kXvqsg==",
+      "version": "0.5.5",
+      "resolved": "https://registry.npmjs.org/babel-plugin-polyfill-regenerator/-/babel-plugin-polyfill-regenerator-0.5.5.tgz",
+      "integrity": "sha512-OJGYZlhLqBh2DDHeqAxWB1XIvr49CxiJ2gIt61/PU55CQK4Z58OzMqjDe1zwQdQk+rBYsRc+1rJmdajM3gimHg==",
       "dependencies": {
-        "@babel/helper-define-polyfill-provider": "^0.4.4"
+        "@babel/helper-define-polyfill-provider": "^0.5.0"
       },
       "peerDependencies": {
         "@babel/core": "^7.4.0 || ^8.0.0-0 <8.0.0"
@@ -5965,9 +5980,9 @@
       }
     },
     "node_modules/caniuse-lite": {
-      "version": "1.0.30001576",
-      "resolved": "https://registry.npmjs.org/caniuse-lite/-/caniuse-lite-1.0.30001576.tgz",
-      "integrity": "sha512-ff5BdakGe2P3SQsMsiqmt1Lc8221NR1VzHj5jXN5vBny9A6fpze94HiVV/n7XRosOlsShJcvMv5mdnpjOGCEgg==",
+      "version": "1.0.30001579",
+      "resolved": "https://registry.npmjs.org/caniuse-lite/-/caniuse-lite-1.0.30001579.tgz",
+      "integrity": "sha512-u5AUVkixruKHJjw/pj9wISlcMpgFWzSrczLZbrqBSxukQixmg0SJ5sZTpvaFvxU0HoQKd4yoyAogyrAz9pzJnA==",
       "funding": [
         {
           "type": "opencollective",
@@ -8427,9 +8442,9 @@
       "integrity": "sha512-36yxDn5H7OFZQla0/jFJmbIKTdZAQHngCedGxiMmpNfEZM0sdEeT+WczLQrjK6D7o2aiyLYDnkw0R3JK0Qv1RQ=="
     },
     "node_modules/follow-redirects": {
-      "version": "1.15.4",
-      "resolved": "https://registry.npmjs.org/follow-redirects/-/follow-redirects-1.15.4.tgz",
-      "integrity": "sha512-Cr4D/5wlrb0z9dgERpUL3LrmPKVDsETIJhaCMeDfuFYcqa5bldGV6wBsAN6X/vxlXQtFBMrXdXxdL8CbDTGniw==",
+      "version": "1.15.5",
+      "resolved": "https://registry.npmjs.org/follow-redirects/-/follow-redirects-1.15.5.tgz",
+      "integrity": "sha512-vSFWUON1B+yAw1VN4xMfxgn5fTUiaOzAJCKBwIIgT/+7CuGy9+r+5gITvP62j3RmaD5Ph65UaERdOSRGUzZtgw==",
       "funding": [
         {
           "type": "individual",
@@ -13960,9 +13975,9 @@
       }
     },
     "node_modules/postcss-modules-local-by-default": {
-      "version": "4.0.3",
-      "resolved": "https://registry.npmjs.org/postcss-modules-local-by-default/-/postcss-modules-local-by-default-4.0.3.tgz",
-      "integrity": "sha512-2/u2zraspoACtrbFRnTijMiQtb4GW4BvatjaG/bCjYQo8kLTdevCUlwuBHx2sCnSyrI3x3qj4ZK1j5LQBgzmwA==",
+      "version": "4.0.4",
+      "resolved": "https://registry.npmjs.org/postcss-modules-local-by-default/-/postcss-modules-local-by-default-4.0.4.tgz",
+      "integrity": "sha512-L4QzMnOdVwRm1Qb8m4x8jsZzKAaPAgrUF1r/hjDR2Xj7R+8Zsf97jAlSQzWtKx5YNiNGN8QxmPFIc/sh+RQl+Q==",
       "dependencies": {
         "icss-utils": "^5.0.0",
         "postcss-selector-parser": "^6.0.2",
@@ -15407,12 +15422,12 @@
       }
     },
     "node_modules/safe-array-concat": {
-      "version": "1.0.1",
-      "resolved": "https://registry.npmjs.org/safe-array-concat/-/safe-array-concat-1.0.1.tgz",
-      "integrity": "sha512-6XbUAseYE2KtOuGueyeobCySj9L4+66Tn6KQMOPQJrAJEowYKW/YR/MGJZl7FdydUdaFu4LYyDZjxf4/Nmo23Q==",
+      "version": "1.1.0",
+      "resolved": "https://registry.npmjs.org/safe-array-concat/-/safe-array-concat-1.1.0.tgz",
+      "integrity": "sha512-ZdQ0Jeb9Ofti4hbt5lX3T2JcAamT9hfzYU1MNB+z/jaEbB6wfFfPIR/zEORmZqobkCCJhSjodobH6WHNmJ97dg==",
       "dependencies": {
-        "call-bind": "^1.0.2",
-        "get-intrinsic": "^1.2.1",
+        "call-bind": "^1.0.5",
+        "get-intrinsic": "^1.2.2",
         "has-symbols": "^1.0.3",
         "isarray": "^2.0.5"
       },
@@ -16611,9 +16626,9 @@
       }
     },
     "node_modules/terser": {
-      "version": "5.26.0",
-      "resolved": "https://registry.npmjs.org/terser/-/terser-5.26.0.tgz",
-      "integrity": "sha512-dytTGoE2oHgbNV9nTzgBEPaqAWvcJNl66VZ0BkJqlvp71IjO8CxdBx/ykCNb47cLnCmCvRZ6ZR0tLkqvZCdVBQ==",
+      "version": "5.27.0",
+      "resolved": "https://registry.npmjs.org/terser/-/terser-5.27.0.tgz",
+      "integrity": "sha512-bi1HRwVRskAjheeYl291n3JC4GgO/Ty4z1nVs5AAsmonJulGxpSektecnNedrwK9C7vpvVtcX3cw00VSLt7U2A==",
       "dependencies": {
         "@jridgewell/source-map": "^0.3.3",
         "acorn": "^8.8.2",

多すぎてどこが影響しているのかわかりません…

まとめ

ESLintの問題だと思って色々試していましたが、そうじゃなかったかもしれません。
まだ初心者なので、実際の原因はわかりませんが、こういう解決方法もあるのだということを学びました。

参考文献

これからはじめるReact実践入門　コンポーネントの基本からNext.jsによるアプリ開発まで