fix: add string flattening to the benchmarks

[ivan-tymoshenko]

The benchmark needs to consume the resulting string

One important detail is that returning a string does not necessarily mean all the work of building it has happened.

FJS returns an unflattened string (a tree of references to smaller string pieces, also called a rope). Native JSON.stringify returns a flat string, with its characters already stored contiguously. Concatenation can therefore look very cheap in a serialization-only benchmark because some copying is deferred.

I updated the benchmark to measure:

const json = serialize(object)
return Buffer.byteLength(json, 'utf8')

This includes serialization and byte length counting, without route or socket overhead. It does not include actual UTF-8 output encoding or the complete HTTP response path.

!! Please take a look at the last 3 bench cases that shows the real world responses.
*all the benchmarks includes unmerged optimization: #886

Results for nodejs 24.

❯ /home/ivan/.nvm/versions/node/v24.15.0/bin/node --expose-gc benchmark/bench-cmp-lib.js --fjs-vs-json

--- creation and first stringify ---
🏆 fast-json-stringify                      x          23,443 ops/sec ±0.25% (23185 runs sampled)

--- array ---
🏆 JSON.stringify                           x          17,366 ops/sec ±0.07% (17353 runs sampled)
   fast-json-stringify [default]            x          14,019 ops/sec ±0.30% (13870 runs sampled)
   fast-json-stringify [json-stringify]     x           8,218 ops/sec ±0.24% (8176 runs sampled)

--- large array ---
🏆 fast-json-stringify [json-stringify]     x             739 ops/sec ±1.05% (731 runs sampled)
   JSON.stringify                           x             730 ops/sec ±1.00% (721 runs sampled)
   fast-json-stringify [default]            x             565 ops/sec ±1.78% (549 runs sampled)

--- long string ---
🏆 JSON.stringify                           x          60,448 ops/sec ±0.25% (59416 runs sampled)
   fast-json-stringify                      x          60,064 ops/sec ±0.14% (58760 runs sampled)

--- short string ---
🏆 fast-json-stringify                      x      23,281,901 ops/sec ±0.02% (22846203 runs sampled)
   JSON.stringify                           x      12,507,725 ops/sec ±0.13% (12328835 runs sampled)

--- obj ---
🏆 JSON.stringify                           x       6,879,736 ops/sec ±4.96% (6646696 runs sampled)
   fast-json-stringify                      x       6,203,188 ops/sec ±19.69% (5533413 runs sampled)

--- date ---
🏆 fast-json-stringify                      x       2,706,689 ops/sec ±0.01% (2701141 runs sampled)
   JSON.stringify                           x       1,550,297 ops/sec ±9.17% (1475123 runs sampled)

--- user profile response ---
🏆 JSON.stringify                           x         853,691 ops/sec ±0.10% (848898 runs sampled)
   fast-json-stringify                      x         435,157 ops/sec ±1.28% (431065 runs sampled)

--- order response ---
🏆 JSON.stringify                           x         166,853 ops/sec ±0.07% (166206 runs sampled)
   fast-json-stringify                      x          82,633 ops/sec ±0.29% (82337 runs sampled)

--- paginated products response ---
🏆 JSON.stringify                           x          22,363 ops/sec ±0.09% (22301 runs sampled)
   fast-json-stringify                      x           9,978 ops/sec ±0.11% (9961 runs sampled)

Results for nodejs 26

❯ /home/ivan/.nvm/versions/node/v26.10.0/bin/node --expose-gc benchmark/bench-cmp-lib.js --fjs-vs-json

--- creation and first stringify ---
🏆 fast-json-stringify                      x          27,081 ops/sec ±0.34% (26624 runs sampled)

--- array ---
🏆 JSON.stringify                           x          33,730 ops/sec ±0.10% (33655 runs sampled)
   fast-json-stringify [default]            x          29,911 ops/sec ±0.12% (29781 runs sampled)
   fast-json-stringify [json-stringify]     x           9,420 ops/sec ±0.38% (9289 runs sampled)

--- large array ---
🏆 JSON.stringify                           x           1,470 ops/sec ±0.46% (1460 runs sampled)
   fast-json-stringify [json-stringify]     x           1,442 ops/sec ±0.46% (1433 runs sampled)
   fast-json-stringify [default]            x           1,270 ops/sec ±1.03% (1247 runs sampled)

--- long string ---
🏆 fast-json-stringify                      x          82,467 ops/sec ±0.13% (82023 runs sampled)
   JSON.stringify                           x          82,200 ops/sec ±0.13% (81773 runs sampled)

--- short string ---
🏆 JSON.stringify                           x      19,301,338 ops/sec ±0.39% (19074572 runs sampled)
   fast-json-stringify                      x      13,977,038 ops/sec ±0.08% (13906866 runs sampled)

--- obj ---
🏆 JSON.stringify                           x      11,413,950 ops/sec ±1.49% (11241825 runs sampled)
   fast-json-stringify                      x       6,479,504 ops/sec ±1.17% (6388408 runs sampled)

--- date ---
🏆 fast-json-stringify                      x       2,993,286 ops/sec ±0.06% (2985258 runs sampled)
   JSON.stringify                           x       1,632,403 ops/sec ±9.90% (1547023 runs sampled)

--- user profile response ---
🏆 JSON.stringify                           x       1,838,032 ops/sec ±6.12% (1746321 runs sampled)
   fast-json-stringify                      x         491,409 ops/sec ±0.12% (489067 runs sampled)

--- order response ---
🏆 JSON.stringify                           x         221,118 ops/sec ±1.03% (218773 runs sampled)
   fast-json-stringify                      x          93,830 ops/sec ±0.41% (93077 runs sampled)

--- paginated products response ---
🏆 JSON.stringify                           x          33,861 ops/sec ±0.11% (33767 runs sampled)
   fast-json-stringify                      x          10,929 ops/sec ±0.88% (10823 runs sampled)

[gurgunday]

lgtm