当项目不仅仅包含.js或.json文件，还包含其他类型文件(如.ts、.vue、.css)作为模块时，配置resolve.extensions可以不必要的文件搜索提高性能。

src/index.ts
import { someFuction } from './module'

someFuction()

src/module.ts

import {otherSomeFuction} from './othermodule'


export const someFuction = otherSomeFuction

src/othermodule.ts
export const otherSomeFuction = () => {
    console.log('otherSomeFuction...')
}

优化前

webpack.config.js

const config = {
    entry: './src/index.ts',
    output: {
        filename: 'main.js'
    },
  mode: 'development',
  resolve: {  
    extensions: [ '.js', '.json', '.jsx', '.tsx', '.ts'], 
  },  
  module: {
    rules: [
      {
        test: /\.ts$/,
        loader: 'ts-loader'
      }
    ]
  }
}

module.exports = config;

优化后

webpack.config.js

const config = {
    entry: './src/index.ts',
    output: {
        filename: 'main.js'
    },
  mode: 'development',
  resolve: {  
    extensions: [ '.ts', '.tsx', '.js', '.jsx', '.json'], 
  },  
  module: {
    rules: [
      {
        test: /\.ts$/,
        loader: 'ts-loader'
      }
    ]
  }
}

module.exports = config;

可以看到优化resolve.extensions的顺序，简单的三个小模块的打包就带来了1508-1467=41ms的性能提升