perf: optimize HeadersList.get

[tsctx]

This relates to...

Rationale

Reducing the number of calls String#toLowerCase showed a 2-fold improvement over main.
However, if no value is present, it will be slightly slower.
See Benchmark

Changes

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Benchmark

// bench.js
const { Headers } = require("./lib/fetch/headers.js");
const { kHeadersList } = require("./lib/core/symbols.js");
const benchmark = require("benchmark");
const suite = new benchmark.Suite();

const headers = new Headers({ "Accept-Encoding": "gzip" });

const headersList = headers[kHeadersList];

suite.add("HeadersList.get - Existing", () => {
  headersList.get("Accept-Encoding");
});

suite.add("HeadersList.get - non Existing", () => {
  headersList.get("Accept-Language");
});

suite.add("Headers.get - Existing", () => {
  headers.get("Accept-Encoding");
});

suite.add("Headers.get - non Existing", () => {
  headers.get("Accept-Language");
});

suite.on("cycle", function (event) {
  console.log(String(event.target));
});

// run async
suite.run({ async: true });

• main

HeadersList.get - Existing x 33,034,921 ops/sec ±3.75% (85 runs sampled)
HeadersList.get - non Existing x 159,049,670 ops/sec ±1.70% (86 runs sampled)
Headers.get - Existing x 4,780,193 ops/sec ±2.16% (77 runs sampled)
Headers.get - non Existing x 4,809,305 ops/sec ±2.08% (80 runs sampled)

• this PR

HeadersList.get - Existing x 62,210,720 ops/sec ±2.63% (82 runs sampled)
HeadersList.get - non Existing x 144,683,562 ops/sec ±2.14% (85 runs sampled)
Headers.get - Existing x 5,080,302 ops/sec ±2.22% (85 runs sampled)
Headers.get - non Existing x 4,974,778 ops/sec ±2.75% (83 runs sampled)```

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (e39a632) 85.54% compared to head (7c520b1) 85.72%.
Report is 85 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2420      +/-   ##
==========================================
+ Coverage   85.54%   85.72%   +0.17%     
==========================================
  Files          76       76              
  Lines        6858     6912      +54     
==========================================
+ Hits         5867     5925      +58     
+ Misses        991      987       -4     

Files	Coverage Δ
index-fetch.js	100.00% <100.00%> (ø)
lib/api/api-stream.js	100.00% <100.00%> (ø)
lib/core/connect.js	80.00% <100.00%> (-1.25%)	⬇️
lib/fetch/body.js	97.68% <100.00%> (ø)
lib/fetch/constants.js	97.22% <100.00%> (+0.67%)	⬆️
lib/fetch/file.js	94.31% <100.00%> (-1.09%)	⬇️
lib/fetch/global.js	100.00% <ø> (ø)
lib/fetch/response.js	97.04% <100.00%> (+1.20%)	⬆️
lib/fetch/util.js	86.53% <100.00%> (+0.71%)	⬆️
lib/fetch/webidl.js	94.50% <100.00%> (-0.03%)	⬇️
... and 10 more

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[mcollina]

lgtm