Extending String.prototype impacts parse5.parse performance #1348

kuhe · 2024-12-06T20:55:12Z

Reported to my team in aws/aws-sdk-js-v3#6182.

Using v7.2.1 of parse5. Node.js v20.5.1.

Why does attempting to extend the native String prototype impact the performance of the parser?

I'm less concerned about mitigating the issue, since we can probably avoid extending the String prototype. I'm mostly curious as to how this is even possible 😕.

"use strict";
const parse5 = require("parse5");

function bench() {
  const A = performance.now();

  const html = `<doctype html><html><head><title>Test</title></head><body><h1>Hello World</h1></body></html>`;

  for (let i = 0; i < 100_000; ++i) {
    const doc = parse5.parse(html);
  }

  const B = performance.now();

  console.log("Done", B - A);
}

bench();
Object.create(String.prototype); // this line shouldn't do anything
bench();

In this example, the line Object.create(String.prototype); should not do anything, since by description this is a read operation of the String prototype. However, leaving this line in causes the second run of bench() to take 20% to 30% longer.

Without the line:

Done 558.624419927597
Done 513.1717121601105 // second run might be faster due to optimization, that is fine.

With the line:

Done 559.1178510189056
Done 694.2957258224487 // second run gets significantly slower

The text was updated successfully, but these errors were encountered:

kuhe · 2024-12-06T21:12:03Z

After some more testing it seems that Object.create(String.prototype) simply de-optimizes string operations in general, and not something specific to parse5. Closing.

kuhe closed this as completed Dec 6, 2024

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Extending String.prototype impacts parse5.parse performance #1348

Extending String.prototype impacts parse5.parse performance #1348

kuhe commented Dec 6, 2024

kuhe commented Dec 6, 2024

Extending String.prototype impacts parse5.parse performance #1348

Extending String.prototype impacts parse5.parse performance #1348

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kuhe commented Dec 6, 2024

kuhe commented Dec 6, 2024