fix: make asString monomorphic and trigger V8 optimization

[nigrosimone]

asString is Polymorphic/Megamorphic because accept more type (string, date, regex, null)

So let’s spend some more time defining this.

• Monomorphic: If function arguments are always the same type.
• Polymorphic: If function arguments are few (four or fewer) different type.
• Megamorphic: If function arguments are more than four type.

In short: functions that are invoked with the same types over and over again are optimized in the JIT compiler, hence faster.

Megamorphism asString (actual code)

short string............................................. x 14,825,973 ops/sec ±4.07% (156 runs sampled)
short string with double quote........................... x 6,743,956 ops/sec ±4.15% (156 runs sampled)
long string without double quotes........................ x 3,662 ops/sec ±5.18% (161 runs sampled)
long string.............................................. x 5,154 ops/sec ±3.17% (183 runs sampled)

Monomorphic asString (my PR)

short string............................................. x 15,750,724 ops/sec ±2.55% (172 runs sampled)
short string with double quote........................... x 6,067,413 ops/sec ±3.97% (158 runs sampled)
long string without double quotes........................ x 19,700 ops/sec ±3.74% (169 runs sampled)
long string.............................................. x 13,170 ops/sec ±1.65% (180 runs sampled)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Uzlopak]

Definetly needs great consensus
@ivan-tymoshenko
@mcollina
@climba03003
@gurgunday
@kibertoad

I propose the necessity of atleast 3 approvals.

[mcollina]

lgtm

[mcollina]

makes 100% sense

[Uzlopak]

LGTM

[kibertoad]

Should types be adjusted accordingly?

[Uzlopak]

Which types?

[kibertoad]

@Uzlopak checked https://github.com/fastify/fast-json-stringify/blob/master/types/index.d.ts, it doesn't seem to be there. so it's an internal only method?

[Uzlopak]

yup, only internal