Module not found: Error: Can

使用 react + webpack 启动一个简单的 react 例子时, 报错 :

Module not found: Error: Can't resolve 'react' in 入口文件
Module not found: Error: Can't resolve 'react-dom' in 入口文件

一. 项目结构:

二. 一些文件内容:
app.jsx:

import React from 'react';
import ReactDom from 'react-dom';

function App() {
    return (
        <div>
            <h1>hello world!</h1>
        </div>
    );
}

const app = document.createElement('div');
document.body.appendChild(app);
ReactDom.render(<App/>,app);

package.json:

{
  "name": "test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "dev":"webpack-dev-server --hot",
    "build": "webpack -w",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "react": "^16.2.0",
    "react-dom": "^16.2.0"
  },
  "devDependencies": {
    "babel-core": "^6.26.0",
    "babel-loader": "^7.1.2",
    "babel-preset-env": "^1.6.1",
    "babel-preset-react": "^6.24.1",
    "clean-webpack-plugin": "^0.1.17",
    "html-webpack-plugin": "^2.30.1",
    "webpack": "^3.8.1",
    "webpack-dev-server": "^2.9.5"
  }
}

webpack.config.js:

var path = require('path');
var webpack = require('webpack');
var HtmlWebpackPlugin = require('html-webpack-plugin');
var CleanWebpackPlugin = require('clean-webpack-plugin');

var ROOT_PATH = path.resolve(__dirname);
var BUILD_PATH = path.resolve(ROOT_PATH, 'build');
var APP_PATH = path.resolve(ROOT_PATH, 'app');

module.exports = {
    entry: {
        app: path.resolve(APP_PATH, 'app.jsx')
    },
    output: {
        path: BUILD_PATH,
        filename: 'bundle_[hash:5].js'
    },

    devServer:{
        contentBase:__dirname
    },

    module: {
        rules: [
            {
                test: /\.jsx?$/,
                loaders: ['babel-loader'],
                include: APP_PATH
            }
        ]
    },

    devtool: 'eval-source-map',

    resolve: {
        extensions: ['.js', '.jsx'],
        modules: [APP_PATH, 'node_modules']
//     之前是这样写的: modules: [APP_PATH] 所以找不到 module 模块
    },

    plugins: [
        new HtmlWebpackPlugin({
            title: 'test App'
        }),
        new CleanWebpackPlugin(['build'])
    ]
};

出现错误的原因: resolve.modules 这个方法使用错了, resolve.modules 告诉 webpack 寻找模块时应该从哪些路径查找, resolve.modules 的默认值就是 ["node_modules"], 我写成这样 modules: [APP_PATH] 就不去 "node_modules" 查找了.

resolve.modules 里的路径可以是相对路径, 也可以是绝对路径. 如果是相对路径, 查找模块时会像 node 查找模块一样, 先从离的最近的 node_modules 目录里查找, 如果没找到, 层层向上继续找, 直到到最外层的根目录的 node_modules. 如果是绝对路径, 就简单多了, 只在指定的路径里查找.