emit-cache-webpack-plugin.js

/* this plugin fixes missing asset caching support in extract-text-webpack-plugin, preventing unnecessary rebuilds */

const { CachedSource, ConcatSource } = require('webpack-sources');
const loaderUtils = require('loader-utils');

module.exports = class {
    apply(compiler) {
        compiler.plugin("compilation", function (compilation) {
            compilation.plugin("optimize-assets", function (assets, callback) {
                if (!assets) {
                    return;
                }
                Object.keys(assets).forEach((asset) => {
                    let source = assets[asset]

                    if (source instanceof CachedSource) {
                        return;
                    }
                    
                    if (!(source instanceof ConcatSource || /\.map$/.test(asset))) {
                        // only process ConcatSource, as this plugin's main purpose is to fix extract-text-webpack-plugin output caching
                        // would probably make sense to generalize and filter out RawSource instead, but source maps are also RawSource in extract-text-webpack-plugin
                        return;
                    }

                    var cacheKey = 'extraCache ' + asset;
                    var chunkHash = loaderUtils.getHashDigest(source.source());
                    if (compilation.cache && compilation.cache[cacheKey] && compilation.cache[cacheKey].hash === chunkHash) {
                        source = compilation.cache[cacheKey].source;
                    } else if (compilation.cache) {
                        compilation.cache[cacheKey] = {
                            hash: chunkHash,
                            source: source = (source instanceof CachedSource ? source : new CachedSource(source))
                        };
                    }

                    assets[asset] = source;
                    compilation.modifyHash(chunkHash);
                })
                callback();
            });
        });
    }
}