babel-plugin-import 是一个用于按需加载组件代码和样式的 babel 插件（原理）
1.通过yarn add babel-plugin-import 安装babel-plugin-import插件
yarn add babel-plugin-import
2.在webpack.config.js中添加如下代码进行配置antd的按需加载
['import',{libraryName: 'antd',style:true}],
整体代码如下：
'use strict';
const fs = require('fs');
const path = require('path');
const webpack = require('webpack');
const resolve = require('resolve');
const PnpWebpackPlugin = require('pnp-webpack-plugin');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const CaseSensitivePathsPlugin = require('case-sensitive-paths-webpack-plugin');
const InlineChunkHtmlPlugin = require('react-dev-utils/InlineChunkHtmlPlugin');
const TerserPlugin = require('terser-webpack-plugin');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
const safePostCssParser = require('postcss-safe-parser');
const ManifestPlugin = require('webpack-manifest-plugin');
const InterpolateHtmlPlugin = require('react-dev-utils/InterpolateHtmlPlugin');
const WorkboxWebpackPlugin = require('workbox-webpack-plugin');
const WatchMissingNodeModulesPlugin = require('react-dev-utils/WatchMissingNodeModulesPlugin');
const ModuleScopePlugin = require('react-dev-utils/ModuleScopePlugin');
const getCSSModuleLocalIdent = require('react-dev-utils/getCSSModuleLocalIdent');
const paths = require('./paths');
const modules = require('./modules');
const getClientEnvironment = require('./env');
const ModuleNotFoundPlugin = require('react-dev-utils/ModuleNotFoundPlugin');
const ForkTsCheckerWebpackPlugin = require('react-dev-utils/ForkTsCheckerWebpackPlugin');
const typescriptFormatter = require('react-dev-utils/typescriptFormatter');
const postcssNormalize = require('postcss-normalize');
const appPackageJson = require(paths.appPackageJson);
// Source maps are resource heavy and can cause out of memory issue for large source files.
const shouldUseSourceMap = process.env.GENERATE_SOURCEMAP !== 'false';
// Some apps do not need the benefits of saving a web request, so not inlining the chunk
// makes for a smoother build process.
const shouldInlineRuntimeChunk = process.env.INLINE_RUNTIME_CHUNK !== 'false';
const imageInlineSizeLimit = parseInt(
process.env.IMAGE_INLINE_SIZE_LIMIT || '10000'
);
// Check if TypeScript is setup
const useTypeScript = fs.existsSync(paths.appTsConfig);
// style files regexes
const cssRegex = /\.css$/;
const cssModuleRegex = /\.module\.css$/;
const sassRegex = /\.(scss|sass)$/;
const sassModuleRegex = /\.module\.(scss|sass)$/;
// This is the production and development configuration.
// It is focused on developer experience, fast rebuilds, and a minimal bundle.
module.exports = function(webpackEnv) {
const isEnvDevelopment = webpackEnv === 'development';
const isEnvProduction = webpackEnv === 'production';
// Variable used for enabling profiling in Production
// passed into alias object. Uses a flag if passed into the build command
const isEnvProductionProfile =
isEnvProduction && process.argv.includes('--profile');
// Webpack uses `publicPath` to determine where the app is being served from.
// It requires a trailing slash, or the file assets will get an incorrect path.
// In development, we always serve from the root. This makes config easier.
const publicPath = isEnvProduction
? paths.servedPath
: isEnvDevelopment && '/';
// Some apps do not use client-side routing with pushState.
// For these, "homepage" can be set to "." to enable relative asset paths.
const shouldUseRelativeAssetPaths = publicPath === './';
// `publicUrl` is just like `publicPath`, but we will provide it to our app
// as %PUBLIC_URL% in `index.html` and `process.env.PUBLIC_URL` in JavaScript.
// Omit trailing slash as %PUBLIC_URL%/xyz looks better than %PUBLIC_URL%xyz.
const publicUrl = isEnvProduction
? publicPath.slice(0, -1)
: isEnvDevelopment && '';
// Get environment variables to inject into our app.
const env = getClientEnvironment(publicUrl);
// common function to get style loaders
const getStyleLoaders = (cssOptions, preProcessor) => {
const loaders = [
isEnvDevelopment && require.resolve('style-loader'),
isEnvProduction && {
loader: MiniCssExtractPlugin.loader,
options: shouldUseRelativeAssetPaths ? { publicPath: '../../' } : {},
},
{
loader: require.resolve('css-loader'),
options: cssOptions,
},
{
// Options for PostCSS as we reference these options twice
// Adds vendor prefixing based on your specified browser support in
// package.json
loader: require.resolve('postcss-loader'),
options: {
// Necessary for external CSS imports to work
// https://github.com/facebook/create-react-app/issues/2677
ident: 'postcss',
plugins: () => [
require('postcss-flexbugs-fixes'),
require('postcss-preset-env')({
autoprefixer: {
flexbox: 'no-2009',
},
stage: 3,
}),
// Adds PostCSS Normalize as the reset css with default options,
// so that it honors browserslist config in package.json
// which in turn let's users customize the target behavior as per their needs.
postcssNormalize(),
],
sourceMap: isEnvProduction && shouldUseSourceMap,
},
},
].filter(Boolean);
if (preProcessor) {
loaders.push(
{
loader: require.resolve('resolve-url-loader'),
options: {
sourceMap: isEnvProduction && shouldUseSourceMap,
},
},
{
loader: require.resolve(preProcessor),
options: {
sourceMap: true,
},
}
);
}
return loaders;
};
return {
mode: isEnvProduction ? 'production' : isEnvDevelopment && 'development',
// Stop compilation early in production
bail: isEnvProduction,
devtool: isEnvProduction
? shouldUseSourceMap
? 'source-map'
: false
: isEnvDevelopment && 'cheap-module-source-map',
// These are the "entry points" to our application.
// This means they will be the "root" imports that are included in JS bundle.
entry: [
// Include an alternative client for WebpackDevServer. A client's job is to
// connect to WebpackDevServer by a socket and get notified about changes.
// When you save a file, the client will either apply hot updates (in case
// of CSS changes), or refresh the page (in case of JS changes). When you
// make a syntax error, this client will display a syntax error overlay.
// Note: instead of the default WebpackDevServer client, we use a custom one
// to bring better experience for Create React App users. You can replace
// the line below with these two lines if you prefer the stock client:
// require.resolve('webpack-dev-server/client') + '?/',
// require.resolve('webpack/hot/dev-server'),
isEnvDevelopment &&
require.resolve('react-dev-utils/webpackHotDevClient'),
// Finally, this is your app's code:
paths.appIndexJs,
// We include the app code last so that if there is a runtime error during
// initialization, it doesn't blow up the WebpackDevServer client, and
// changing JS code would still trigger a refresh.
].filter(Boolean),
output: {
// The build folder.
path: isEnvProduction ? paths.appBuild : undefined,
// Add /* filename */ comments to generated require()s in the output.
pathinfo: isEnvDevelopment,
// There will be one main bundle, and one file per asynchronous chunk.
// In development, it does not produce real files.
filename: isEnvProduction
? 'static/js/[name].[contenthash:8].js'
: isEnvDevelopment && 'static/js/bundle.js',
// TODO: remove this when upgrading to webpack 5
futureEmitAssets: true,
// There are also additional JS chunk files if you use code splitting.
chunkFilename: isEnvProduction
? 'static/js/[name].[contenthash:8].chunk.js'
: isEnvDevelopment && 'static/js/[name].chunk.js',
// We inferred the "public path" (such as / or /my-project) from homepage.
// We use "/" in development.
publicPath: publicPath,
// Point sourcemap entries to original disk location (format as URL on Windows)
devtoolModuleFilenameTemplate: isEnvProduction
? info =>
path
.relative(paths.appSrc, info.absoluteResourcePath)
.replace(/\\/g, '/')
: isEnvDevelopment &&
(info => path.resolve(info.absoluteResourcePath).replace(/\\/g, '/')),
// Prevents conflicts when multiple Webpack runtimes (from different apps)
// are used on the same page.
jsonpFunction: `webpackJsonp${appPackageJson.name}`,
// this defaults to 'window', but by setting it to 'this' then
// module chunks which are built will work in web workers as well.
globalObject: 'this',
},
optimization: {
minimize: isEnvProduction,
minimizer: [
// This is only used in production mode
new TerserPlugin({
terserOptions: {
parse: {
// We want terser to parse ecma 8 code. However, we don't want it
// to apply any minification steps that turns valid ecma 5 code
// into invalid ecma 5 code. This is why the 'compress' and 'output'
// sections only apply transformations that are ecma 5 safe
// https://github.com/facebook/create-react-app/pull/4234
ecma: 8,
},
compress: {
ecma: 5,
warnings: false,
// Disabled because of an issue with Uglify breaking seemingly valid code:
// https://github.com/facebook/create-react-app/issues/2376
// Pending further investigation:
// https://github.com/mishoo/UglifyJS2/issues/2011
comparisons: false,
// Disabled because of an issue with Terser breaking valid code:
// https://github.com/facebook/create-react-app/issues/5250
// Pending further investigation:
// https://github.com/terser-js/terser/issues/120
inline: 2,
},
mangle: {
safari10: true,
},
// Added for profiling in devtools
keep_classnames: isEnvProductionProfile,
keep_fnames: isEnvProductionProfile,
output: {
ecma: 5,
comments: false,
// Turned on because emoji and regex is not minified properly using default
// https://github.com/facebook/create-react-app/issues/2488
ascii_only: true,
},
},
sourceMap: shouldUseSourceMap,
}),
// This is only used in production mode
new OptimizeCSSAssetsPlugin({
cssProcessorOptions: {
parser: safePostCssParser,
map: shouldUseSourceMap
? {
// `inline: false` forces the sourcemap to be output into a
// separate file
inline: false,
// `annotation: true` appends the sourceMappingURL to the end of
// the css file, helping the browser find the sourcemap
annotation: true,
}
: false,
},
cssProcessorPluginOptions: {
preset: ['default', { minifyFontValues: { removeQuotes: false } }]
}
}),
],
// Automatically split vendor and commons
// https://twitter.com/wSokra/status/969633336732905474
// https://medium.com/webpack/webpack-4-code-splitting-chunk-graph-and-the-splitchunks-optimization-be739a861366
splitChunks: {
chunks: 'all',
name: false,
},
// Keep the runtime chunk separated to enable long term caching
// https://twitter.com/wSokra/status/969679223278505985
// https://github.com/facebook/create-react-app/issues/5358
runtimeChunk: {
name: entrypoint => `runtime-${entrypoint.name}`,
},
},
resolve: {
// This allows you to set a fallback for where Webpack should look for modules.
// We placed these paths second because we want `node_modules` to "win"
// if there are any conflicts. This matches Node resolution mechanism.
// https://github.com/facebook/create-react-app/issues/253
modules: ['node_modules', paths.appNodeModules].concat(
modules.additionalModulePaths || []
),
// These are the reasonable defaults supported by the Node ecosystem.
// We also include JSX as a common component filename extension to support
// some tools, although we do not recommend using it, see:
// https://github.com/facebook/create-react-app/issues/290
// `web` extension prefixes have been added for better support
// for React Native Web.
extensions: paths.moduleFileExtensions
.map(ext => `.${ext}`)
.filter(ext => useTypeScript || !ext.includes('ts')),
alias: {
// Support React Native Web
// https://www.smashingmagazine.com/2016/08/a-glimpse-into-the-future-with-react-native-for-web/
'react-native': 'react-native-web',
// Allows for better profiling with ReactDevTools
...(isEnvProductionProfile && {
'react-dom$': 'react-dom/profiling',
'scheduler/tracing': 'scheduler/tracing-profiling',
}),
...(modules.webpackAliases || {}),
},
plugins: [
// Adds support for installing with Plug'n'Play, leading to faster installs and adding
// guards against forgotten dependencies and such.
PnpWebpackPlugin,
// Prevents users from importing files from outside of src/ (or node_modules/).
// This often causes confusion because we only process files within src/ with babel.
// To fix this, we prevent you from importing files out of src/ -- if you'd like to,
// please link the files into your node_modules/ and let module-resolution kick in.
// Make sure your source files are compiled, as they will not be processed in any way.
new ModuleScopePlugin(paths.appSrc, [paths.appPackageJson]),
],
},
resolveLoader: {
plugins: [
// Also related to Plug'n'Play, but this time it tells Webpack to load its loaders
// from the current package.
PnpWebpackPlugin.moduleLoader(module),
],
},
module: {
strictExportPresence: true,
rules: [
// Disable require.ensure as it's not a standard language feature.
{ parser: { requireEnsure: false } },
// First, run the linter.
// It's important to do this before Babel processes the JS.
{
test: /\.(js|mjs|jsx|ts|tsx)$/,
enforce: 'pre',
use: [
{
options: {
cache: true,
formatter: require.resolve('react-dev-utils/eslintFormatter'),
eslintPath: require.resolve('eslint'),
resolvePluginsRelativeTo: __dirname,
},
loader: require.resolve('eslint-loader'),
},
],
include: paths.appSrc,
},
{
// "oneOf" will traverse all following loaders until one will
// match the requirements. When no loader matches it will fall
// back to the "file" loader at the end of the loader list.
oneOf: [
// "url" loader works like "file" loader except that it embeds assets
// smaller than specified limit in bytes as data URLs to avoid requests.
// A missing `test` is equivalent to a match.
{
test: [/\.bmp$/, /\.gif$/, /\.jpe?g$/, /\.png$/],
loader: require.resolve('url-loader'),
options: {
limit: imageInlineSizeLimit,
name: 'static/media/[name].[hash:8].[ext]',
},
},
// Process application JS with Babel.
// The preset includes JSX, Flow, TypeScript, and some ESnext features.
//解析js文件
{
test: /\.(js|mjs|jsx|ts|tsx)$/,
include: paths.appSrc,
loader: require.resolve('babel-loader'),
options: {
customize: require.resolve(
'babel-preset-react-app/webpack-overrides'
),
plugins: [
//添加此代码实现antd的按需加载
['import',{libraryName: 'antd',style:true}],
//添加此代码实现antd的按需加载
[
require.resolve('babel-plugin-named-asset-import'),
{
loaderMap: {
svg: {
ReactComponent:
'@svgr/webpack?-svgo,+titleProp,+ref![path]',
},
},
},
],
],
// This is a feature of `babel-loader` for webpack (not Babel itself).
// It enables caching results in ./node_modules/.cache/babel-loader/
// directory for faster rebuilds.
cacheDirectory: true,
// See #6846 for context on why cacheCompression is disabled
cacheCompression: false,
compact: isEnvProduction,
},
},
// Process any JS outside of the app with Babel.
// Unlike the application JS, we only compile the standard ES features.
{
test: /\.(js|mjs)$/,
exclude: /@babel(?:\/|\\{1,2})runtime/,
loader: require.resolve('babel-loader'),
options: {
babelrc: false,
configFile: false,
compact: false,
presets: [
[
require.resolve('babel-preset-react-app/dependencies'),
{ helpers: true },
],
],
cacheDirectory: true,
// See #6846 for context on why cacheCompression is disabled
cacheCompression: false,
// Babel sourcemaps are needed for debugging into node_modules
// code. Without the options below, debuggers like VSCode
// show incorrect code and set breakpoints on the wrong lines.
sourceMaps: shouldUseSourceMap,
inputSourceMap: shouldUseSourceMap,
},
},
// "postcss" loader applies autoprefixer to our CSS.
// "css" loader resolves paths in CSS and adds assets as dependencies.
// "style" loader turns CSS into JS modules that inject <style> tags.
// In production, we use MiniCSSExtractPlugin to extract that CSS
// to a file, but in development "style" loader enables hot editing
// of CSS.
// By default we support CSS Modules with the extension .module.css
{
test: cssRegex,
exclude: cssModuleRegex,
use: getStyleLoaders({
importLoaders: 1,
sourceMap: isEnvProduction && shouldUseSourceMap,
}),
// Don't consider CSS imports dead code even if the
// containing package claims to have no side effects.
// Remove this when webpack adds a warning or an error for this.
// See https://github.com/webpack/webpack/issues/6571
sideEffects: true,
},
//添加此内容可以解析less文件
{
test: /\.less$/,
use: [
require.resolve('style-loader'),
{
loader: require.resolve('css-loader'),
options: {
importLoaders: 1,
},
},
{
loader: require.resolve('postcss-loader'),
options: {
// Necessary for external CSS imports to work
// https://github.com/facebookincubator/create-react-app/issues/2677
ident: 'postcss',
plugins: () => [
require('postcss-flexbugs-fixes'),
autoprefixer({
browsers: [
'>1%',
'last 4 versions',
'Firefox ESR',
'not ie < 9', // React doesn't support IE8 anyway
],
flexbox: 'no-2009',
}),
],
},
},
{
loader:require.resolve('less-loader')
}
],
},
//添加此内容可以解析less文件
// Adds support for CSS Modules (https://github.com/css-modules/css-modules)
// using the extension .module.css
{
test: cssModuleRegex,
use: getStyleLoaders({
importLoaders: 1,
sourceMap: isEnvProduction && shouldUseSourceMap,
modules: {
getLocalIdent: getCSSModuleLocalIdent,
},
}),
},
// Opt-in support for SASS (using .scss or .sass extensions).
// By default we support SASS Modules with the
// extensions .module.scss or .module.sass
{
test: sassRegex,
exclude: sassModuleRegex,
use: getStyleLoaders(
{
importLoaders: 3,
sourceMap: isEnvProduction && shouldUseSourceMap,
},
'sass-loader'
),
// Don't consider CSS imports dead code even if the
// containing package claims to have no side effects.
// Remove this when webpack adds a warning or an error for this.
// See https://github.com/webpack/webpack/issues/6571
sideEffects: true,
},
// Adds support for CSS Modules, but using SASS
// using the extension .module.scss or .module.sass
{
test: sassModuleRegex,
use: getStyleLoaders(
{
importLoaders: 3,
sourceMap: isEnvProduction && shouldUseSourceMap,
modules: {
getLocalIdent: getCSSModuleLocalIdent,
},
},
'sass-loader'
),
},
// "file" loader makes sure those assets get served by WebpackDevServer.
// When you `import` an asset, you get its (virtual) filename.
// In production, they would get copied to the `build` folder.
// This loader doesn't use a "test" so it will catch all modules
// that fall through the other loaders.
{
loader: require.resolve('file-loader'),
// Exclude `js` files to keep "css" loader working as it injects
// its runtime that would otherwise be processed through "file" loader.
// Also exclude `html` and `json` extensions so they get processed
// by webpacks internal loaders.
exclude: [/\.(js|mjs|jsx|ts|tsx)$/, /\.html$/, /\.json$/],
options: {
name: 'static/media/[name].[hash:8].[ext]',
},
},
// ** STOP ** Are you adding a new loader?
// Make sure to add the new loader(s) before the "file" loader.
],
},
],
},
plugins: [
// Generates an `index.html` file with the <script> injected.
new HtmlWebpackPlugin(
Object.assign(
{},
{
inject: true,
template: paths.appHtml,
},
isEnvProduction
? {
minify: {
removeComments: true,
collapseWhitespace: true,
removeRedundantAttributes: true,
useShortDoctype: true,
removeEmptyAttributes: true,
removeStyleLinkTypeAttributes: true,
keepClosingSlash: true,
minifyJS: true,
minifyCSS: true,
minifyURLs: true,
},
}
: undefined
)
),
// Inlines the webpack runtime script. This script is too small to warrant
// a network request.
// https://github.com/facebook/create-react-app/issues/5358
isEnvProduction &&
shouldInlineRuntimeChunk &&
new InlineChunkHtmlPlugin(HtmlWebpackPlugin, [/runtime-.+[.]js/]),
// Makes some environment variables available in index.html.
// The public URL is available as %PUBLIC_URL% in index.html, e.g.:
// <link rel="icon" href="%PUBLIC_URL%/favicon.ico">
// In production, it will be an empty string unless you specify "homepage"
// in `package.json`, in which case it will be the pathname of that URL.
// In development, this will be an empty string.
new InterpolateHtmlPlugin(HtmlWebpackPlugin, env.raw),
// This gives some necessary context to module not found errors, such as
// the requesting resource.
new ModuleNotFoundPlugin(paths.appPath),
// Makes some environment variables available to the JS code, for example:
// if (process.env.NODE_ENV === 'production') { ... }. See `./env.js`.
// It is absolutely essential that NODE_ENV is set to production
// during a production build.
// Otherwise React will be compiled in the very slow development mode.
new webpack.DefinePlugin(env.stringified),
// This is necessary to emit hot updates (currently CSS only):
isEnvDevelopment && new webpack.HotModuleReplacementPlugin(),
// Watcher doesn't work well if you mistype casing in a path so we use
// a plugin that prints an error when you attempt to do this.
// See https://github.com/facebook/create-react-app/issues/240
isEnvDevelopment && new CaseSensitivePathsPlugin(),
// If you require a missing module and then `npm install` it, you still have
// to restart the development server for Webpack to discover it. This plugin
// makes the discovery automatic so you don't have to restart.
// See https://github.com/facebook/create-react-app/issues/186
isEnvDevelopment &&
new WatchMissingNodeModulesPlugin(paths.appNodeModules),
isEnvProduction &&
new MiniCssExtractPlugin({
// Options similar to the same options in webpackOptions.output
// both options are optional
filename: 'static/css/[name].[contenthash:8].css',
chunkFilename: 'static/css/[name].[contenthash:8].chunk.css',
}),
// Generate an asset manifest file with the following content:
// - "files" key: Mapping of all asset filenames to their corresponding
// output file so that tools can pick it up without having to parse
// `index.html`
// - "entrypoints" key: Array of files which are included in `index.html`,
// can be used to reconstruct the HTML if necessary
new ManifestPlugin({
fileName: 'asset-manifest.json',
publicPath: publicPath,
generate: (seed, files, entrypoints) => {
const manifestFiles = files.reduce((manifest, file) => {
manifest[file.name] = file.path;
return manifest;
}, seed);
const entrypointFiles = entrypoints.main.filter(
fileName => !fileName.endsWith('.map')
);
return {
files: manifestFiles,
entrypoints: entrypointFiles,
};
},
}),
// Moment.js is an extremely popular library that bundles large locale files
// by default due to how Webpack interprets its code. This is a practical
// solution that requires the user to opt into importing specific locales.
// https://github.com/jmblog/how-to-optimize-momentjs-with-webpack
// You can remove this if you don't use Moment.js:
new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/),
// Generate a service worker script that will precache, and keep up to date,
// the HTML & assets that are part of the Webpack build.
isEnvProduction &&
new WorkboxWebpackPlugin.GenerateSW({
clientsClaim: true,
exclude: [/\.map$/, /asset-manifest\.json$/],
importWorkboxFrom: 'cdn',
navigateFallback: publicUrl + '/index.html',
navigateFallbackBlacklist: [
// Exclude URLs starting with /_, as they're likely an API call
new RegExp('^/_'),
// Exclude any URLs whose last part seems to be a file extension
// as they're likely a resource and not a SPA route.
// URLs containing a "?" character won't be blacklisted as they're likely
// a route with query params (e.g. auth callbacks).
new RegExp('/[^/?]+\\.[^/]+$'),
],
}),
// TypeScript type checking
useTypeScript &&
new ForkTsCheckerWebpackPlugin({
typescript: resolve.sync('typescript', {
basedir: paths.appNodeModules,
}),
async: isEnvDevelopment,
useTypescriptIncrementalApi: true,
checkSyntacticErrors: true,
resolveModuleNameModule: process.versions.pnp
? `${__dirname}/pnpTs.js`
: undefined,
resolveTypeReferenceDirectiveModule: process.versions.pnp
? `${__dirname}/pnpTs.js`
: undefined,
tsconfig: paths.appTsConfig,
reportFiles: [
'**',
'!**/__tests__/**',
'!**/?(*.)(spec|test).*',
'!**/src/setupProxy.*',
'!**/src/setupTests.*',
],
silent: true,
// The formatter is invoked directly in WebpackDevServerUtils during development
formatter: isEnvProduction ? typescriptFormatter : undefined,
}),
].filter(Boolean),
// Some libraries import Node modules but don't use them in the browser.
// Tell Webpack to provide empty mocks for them so importing them works.
node: {
module: 'empty',
dgram: 'empty',
dns: 'mock',
fs: 'empty',
http2: 'empty',
net: 'empty',
tls: 'empty',
child_process: 'empty',
},
// Turn off performance processing because we utilize
// our own hints via the FileSizeReporter
performance: false,
};
};
来源：CSDN
作者：chunchun1230
链接：https://blog.csdn.net/chunchun1230/article/details/104310845