feat: add rel preload support - useRelPreload option

Author: jgillespie

dist/index.js

@@ -115,7 +115,8 @@ class CssModuleFactory {
 class MiniCssExtractPlugin {
   constructor(options) {
     this.options = Object.assign({
-      filename: '[name].css'
+      filename: '[name].css',
+      useRelPreload: false
     }, options);
     if (!this.options.chunkFilename) {
       const { filename } = this.options;
@@ -253,7 +254,9 @@ class MiniCssExtractPlugin {
             },
             contentHashType: MODULE_TYPE
           });
-          return Template.asString([source, '', `// ${pluginName} CSS loading`, 'var supportsPreload = (function() { try { return document.createElement("link").relList.supports("preload"); } catch(e) { return false; }}());', `var cssChunks = ${JSON.stringify(chunkMap)};`, 'if(installedCssChunks[chunkId]) promises.push(installedCssChunks[chunkId]);', 'else if(installedCssChunks[chunkId] !== 0 && cssChunks[chunkId]) {', Template.indent(['promises.push(installedCssChunks[chunkId] = new Promise(function(resolve, reject) {', Template.indent([`var href = ${linkHrefPath};`, `var fullhref = ${mainTemplate.requireFn}.p + href;`, 'var existingLinkTags = document.getElementsByTagName("link");', 'for(var i = 0; i < existingLinkTags.length; i++) {', Template.indent(['var tag = existingLinkTags[i];', 'var dataHref = tag.getAttribute("data-href") || tag.getAttribute("href");', 'if((tag.rel === "stylesheet" || tag.rel === "preload") && (dataHref === href || dataHref === fullhref)) return resolve();']), '}', 'var existingStyleTags = document.getElementsByTagName("style");', 'for(var i = 0; i < existingStyleTags.length; i++) {', Template.indent(['var tag = existingStyleTags[i];', 'var dataHref = tag.getAttribute("data-href");', 'if(dataHref === href || dataHref === fullhref) return resolve();']), '}', 'var linkTag = document.createElement("link");', 'linkTag.rel = supportsPreload ? "preload": "stylesheet";', 'supportsPreload ? linkTag.as = "style" : linkTag.type = "text/css";', 'linkTag.onload = resolve;', 'linkTag.onerror = function(event) {', Template.indent(['var request = event && event.target && event.target.src || fullhref;', 'var err = new Error("Loading CSS chunk " + chunkId + " failed.\\n(" + request + ")");', 'err.request = request;', 'delete installedCssChunks[chunkId]', 'linkTag.parentNode.removeChild(linkTag)', 'reject(err);']), '};', 'linkTag.href = fullhref;', crossOriginLoading ? Template.asString([`if (linkTag.href.indexOf(window.location.origin + '/') !== 0) {`, Template.indent(`linkTag.crossOrigin = ${JSON.stringify(crossOriginLoading)};`), '}']) : '', 'var head = document.getElementsByTagName("head")[0];', 'head.appendChild(linkTag);']), '}).then(function() {', Template.indent(['installedCssChunks[chunkId] = 0;', 'if(supportsPreload) {', Template.indent(['var execLinkTag = document.createElement("link");', `execLinkTag.href =  ${mainTemplate.requireFn}.p + ${linkHrefPath};`, 'execLinkTag.rel = "stylesheet";', 'execLinkTag.type = "text/css";', 'document.body.appendChild(execLinkTag);']), '}']), '}));']), '}']);
+
+          const supportsPreload = this.options.useRelPreload ? '(function() { try { return document.createElement("link").relList.supports("preload"); } catch(e) { return false; }}());' : 'false;';
+          return Template.asString([source, '', `// ${pluginName} CSS loading`, `var supportsPreload = ${supportsPreload}`, `var cssChunks = ${JSON.stringify(chunkMap)};`, 'if(installedCssChunks[chunkId]) promises.push(installedCssChunks[chunkId]);', 'else if(installedCssChunks[chunkId] !== 0 && cssChunks[chunkId]) {', Template.indent(['promises.push(installedCssChunks[chunkId] = new Promise(function(resolve, reject) {', Template.indent([`var href = ${linkHrefPath};`, `var fullhref = ${mainTemplate.requireFn}.p + href;`, 'var existingLinkTags = document.getElementsByTagName("link");', 'for(var i = 0; i < existingLinkTags.length; i++) {', Template.indent(['var tag = existingLinkTags[i];', 'var dataHref = tag.getAttribute("data-href") || tag.getAttribute("href");', 'if((tag.rel === "stylesheet" || tag.rel === "preload") && (dataHref === href || dataHref === fullhref)) return resolve();']), '}', 'var existingStyleTags = document.getElementsByTagName("style");', 'for(var i = 0; i < existingStyleTags.length; i++) {', Template.indent(['var tag = existingStyleTags[i];', 'var dataHref = tag.getAttribute("data-href");', 'if(dataHref === href || dataHref === fullhref) return resolve();']), '}', 'var linkTag = document.createElement("link");', 'linkTag.rel = supportsPreload ? "preload": "stylesheet";', 'supportsPreload ? linkTag.as = "style" : linkTag.type = "text/css";', 'linkTag.onload = resolve;', 'linkTag.onerror = function(event) {', Template.indent(['var request = event && event.target && event.target.src || fullhref;', 'var err = new Error("Loading CSS chunk " + chunkId + " failed.\\n(" + request + ")");', 'err.request = request;', 'delete installedCssChunks[chunkId]', 'linkTag.parentNode.removeChild(linkTag)', 'reject(err);']), '};', 'linkTag.href = fullhref;', crossOriginLoading ? Template.asString([`if (linkTag.href.indexOf(window.location.origin + '/') !== 0) {`, Template.indent(`linkTag.crossOrigin = ${JSON.stringify(crossOriginLoading)};`), '}']) : '', 'var head = document.getElementsByTagName("head")[0];', 'head.appendChild(linkTag);']), '}).then(function() {', Template.indent(['installedCssChunks[chunkId] = 0;', 'if(supportsPreload) {', Template.indent(['var execLinkTag = document.createElement("link");', `execLinkTag.href =  ${mainTemplate.requireFn}.p + ${linkHrefPath};`, 'execLinkTag.rel = "stylesheet";', 'execLinkTag.type = "text/css";', 'document.body.appendChild(execLinkTag);']), '}']), '}));']), '}']);
         }
         return source;
       });

src/index.js

@@ -113,6 +113,7 @@ class MiniCssExtractPlugin {
     this.options = Object.assign(
       {
         filename: '[name].css',
+        useRelPreload: false,
       },
       options
     );
@@ -323,11 +324,15 @@ class MiniCssExtractPlugin {
                 contentHashType: MODULE_TYPE,
               }
             );
+
+            const supportsPreload = this.options.useRelPreload
+              ? '(function() { try { return document.createElement("link").relList.supports("preload"); } catch(e) { return false; }}());'
+              : 'false;';
             return Template.asString([
               source,
               '',
               `// ${pluginName} CSS loading`,
-              'var supportsPreload = (function() { try { return document.createElement("link").relList.supports("preload"); } catch(e) { return false; }}());',
+              `var supportsPreload = ${supportsPreload}`,
               `var cssChunks = ${JSON.stringify(chunkMap)};`,
               'if(installedCssChunks[chunkId]) promises.push(installedCssChunks[chunkId]);',
               'else if(installedCssChunks[chunkId] !== 0 && cssChunks[chunkId]) {',