(perf) Add comments to blanked script/style tags (#654)

Svelte's parse method is very slow when scripts contain lots of whitespace.
Instead of changing the regex in svelte, we just stop making too much white space by introducing /**/ comments at the end of lines when blanking out the script tag.

#253

Author: halfnelson

packages/svelte2tsx/src/utils/htmlxparser.ts

@@ -82,10 +82,13 @@ function blankVerbatimContent(htmlx: string, verbatimElements: Node[]) {
     for (const node of verbatimElements) {
         const content = node.content;
         if (content) {
-            output =
-                output.substring(0, content.start) +
-                output.substring(content.start, content.end).replace(/[^\n]/g, ' ') +
-                output.substring(content.end);
+            output = htmlx.substring(0, content.start) +
+            htmlx.substring(content.start, content.end)
+                // blank out the content
+                .replace(/[^\n]/g, ' ')
+                // excess blank space can make the svelte parser very slow (sec->min). break it up with comments (works in style/script)
+                .replace(/[^\n][^\n][^\n][^\n]\n/g, '/**/\n') +
+            htmlx.substring(content.end);
         }
     }
     return output;

packages/svelte2tsx/test/htmlxparser/index.js

@@ -0,0 +1,15 @@
+let converter = require('../build/htmlxtojsx')
+let fs = require('fs')
+let assert = require('assert')
+
+describe('htmlxparser', () => {
+    let content = fs.readFileSync(`${__dirname}/large.svelte`, {encoding: 'utf8'});
+    
+    it('parses in a reasonable time', () => {
+        const start = new Date();
+        converter.htmlx2jsx(content);
+        const elapsed = new Date() - start;
+        assert(elapsed <= 1000, `Parsing took ${elapsed} ms, which was longer than 1000ms`);
+    })
+    
+});