React Package.jsonの Proxyの設定

前提

expressでapiを作成 (localhost:4000で起動)
reactでaxiosを使ってapiを叩く

困っていること

Proxyを設定した時と設定せずにfull pathでapiを叩いた時で返ってくるデータが違う

参考コード

APIはすごくシンプルで簡素な物

js:API
1app.get("/", (req, res) => {
2  return res.json({ msg: "rootです。" });
3});

Full Pathで叩いてみる(途中省略)

js
1axios.get("http://localhost:4000").then((res) => {
2    console.log(res.data);
3  });
4
5// 結果
6{msg: 'rootです。'}

Package.jsonにproxyを設定して省略する

json:Package.json
1"proxy": "http://localhost:4000"

js
1// Proxyを設定したので省略して叩く
2axios.get("/").then((res) => {
3    console.log(res.data);
4  });

js
1// 結果
2
3<!DOCTYPE html>
4<html lang="en">
5  <head>
6    <meta charset="utf-8" />
7    <link rel="icon" href="/favicon.ico" />
8    <meta name="viewport" content="width=device-width, initial-scale=1" />
9    <meta name="theme-color" content="#000000" />
10    <meta
11      name="description"
12      content="Web site created using create-react-app"
13    />
14    <link rel="apple-touch-icon" href="/logo192.png" />
15    <!--
16      manifest.json provides metadata used when your web app is installed on a
17      user's mobile device or desktop. See https://developers.google.com/web/fundamentals/web-app-manifest/
18    -->
19    <link rel="manifest" href="/manifest.json" />
20    <!--
21      Notice the use of  in the tags above.
22      It will be replaced with the URL of the `public` folder during the build.
23      Only files inside the `public` folder can be referenced from the HTML.
24
25      Unlike "/favicon.ico" or "favicon.ico", "/favicon.ico" will
26      work correctly both with client-side routing and a non-root public URL.
27      Learn how to configure a non-root public URL by running `npm run build`.
28    -->
29    <title>React App</title>
30  <script defer src="/static/js/bundle.js"></script></head>
31  <body>
32    <noscript>You need to enable JavaScript to run this app.</noscript>
33    <div id="root"></div>
34    <!--
35      This HTML file is a template.
36      If you open it directly in the browser, you will see an empty page.
37
38      You can add webfonts, meta tags, or analytics to this file.
39      The build step will place the bundled scripts into the <body> tag.
40
41      To begin the development, run `npm start` or `yarn start`.
42      To create a production bundle, use `npm run build` or `yarn build`.
43    -->
44  </body>
45</html>