Class Field initialization time

field-init-perf.js

class Classic_Constructor_Assignments {
    constructor() {
        this.verification=1;
        
        this._00=1; this._01=1; this._02=1; this._03=1; this._04=1; this._05=1; this._06=1; this._07=1; this._08=1; this._09=1;
        this._10=1; this._11=1; this._12=1; this._13=1; this._14=1; this._15=1; this._16=1; this._17=1; this._18=1; this._19=1;
        this._20=1; this._21=1; this._22=1; this._23=1; this._24=1; this._25=1; this._26=1; this._27=1; this._28=1; this._29=1;
        this._30=1; this._31=1; this._32=1; this._33=1; this._34=1; this._35=1; this._36=1; this._37=1; this._38=1; this._39=1;
        this._40=1; this._41=1; this._42=1; this._43=1; this._44=1; this._45=1; this._46=1; this._47=1; this._48=1; this._49=1;
        this._50=1; this._51=1; this._52=1; this._53=1; this._54=1; this._55=1; this._56=1; this._57=1; this._58=1; this._59=1;
        this._60=1; this._61=1; this._62=1; this._63=1; this._64=1; this._65=1; this._66=1; this._67=1; this._68=1; this._69=1;
        this._70=1; this._71=1; this._72=1; this._73=1; this._74=1; this._75=1; this._76=1; this._77=1; this._78=1; this._79=1;
        this._80=1; this._81=1; this._82=1; this._83=1; this._84=1; this._85=1; this._86=1; this._87=1; this._88=1; this._89=1;
        this._90=1; this._91=1; this._92=1; this._93=1; this._94=1; this._95=1; this._96=1; this._97=1; this._98=1; this._99=1;
    }
}

class ES2022_Public_Fields {
    verification=1;
    
    _00=1; _01=1; _02=1; _03=1; _04=1; _05=1; _06=1; _07=1; _08=1; _09=1;
    _10=1; _11=1; _12=1; _13=1; _14=1; _15=1; _16=1; _17=1; _18=1; _19=1;
    _20=1; _21=1; _22=1; _23=1; _24=1; _25=1; _26=1; _27=1; _28=1; _29=1;
    _30=1; _31=1; _32=1; _33=1; _34=1; _35=1; _36=1; _37=1; _38=1; _39=1;
    _40=1; _41=1; _42=1; _43=1; _44=1; _45=1; _46=1; _47=1; _48=1; _49=1;
    _50=1; _51=1; _52=1; _53=1; _54=1; _55=1; _56=1; _57=1; _58=1; _59=1;
    _60=1; _61=1; _62=1; _63=1; _64=1; _65=1; _66=1; _67=1; _68=1; _69=1;
    _70=1; _71=1; _72=1; _73=1; _74=1; _75=1; _76=1; _77=1; _78=1; _79=1;
    _80=1; _81=1; _82=1; _83=1; _84=1; _85=1; _86=1; _87=1; _88=1; _89=1;
    _90=1; _91=1; _92=1; _93=1; _94=1; _95=1; _96=1; _97=1; _98=1; _99=1;
}

class ES2022_Private_Fields {
    verification=1;
    
    #_00=1; #_01=1; #_02=1; #_03=1; #_04=1; #_05=1; #_06=1; #_07=1; #_08=1; #_09=1;
    #_10=1; #_11=1; #_12=1; #_13=1; #_14=1; #_15=1; #_16=1; #_17=1; #_18=1; #_19=1;
    #_20=1; #_21=1; #_22=1; #_23=1; #_24=1; #_25=1; #_26=1; #_27=1; #_28=1; #_29=1;
    #_30=1; #_31=1; #_32=1; #_33=1; #_34=1; #_35=1; #_36=1; #_37=1; #_38=1; #_39=1;
    #_40=1; #_41=1; #_42=1; #_43=1; #_44=1; #_45=1; #_46=1; #_47=1; #_48=1; #_49=1;
    #_50=1; #_51=1; #_52=1; #_53=1; #_54=1; #_55=1; #_56=1; #_57=1; #_58=1; #_59=1;
    #_60=1; #_61=1; #_62=1; #_63=1; #_64=1; #_65=1; #_66=1; #_67=1; #_68=1; #_69=1;
    #_70=1; #_71=1; #_72=1; #_73=1; #_74=1; #_75=1; #_76=1; #_77=1; #_78=1; #_79=1;
    #_80=1; #_81=1; #_82=1; #_83=1; #_84=1; #_85=1; #_86=1; #_87=1; #_88=1; #_89=1;
    #_90=1; #_91=1; #_92=1; #_93=1; #_94=1; #_95=1; #_96=1; #_97=1; #_98=1; #_99=1;
}


// Benchmarking

const ITERATIONS = 100_000;
console.log(`Time to construct ${ITERATIONS} instances that each initialize 100 fields using different techniques.`);

function bench (testClass) {
    var start = performance.now();
    var acc = 0;
    for (var i=0; i<ITERATIONS; i++) {
        const instance = new testClass()
        acc += instance.verification;
    }
    var duration = performance.now() - start;
    
    console.log(`${testClass.name} took ${Math.ceil(duration)}ms  (honest:${acc===ITERATIONS})`);
}

bench(Classic_Constructor_Assignments);
bench(ES2022_Public_Fields);
bench(ES2022_Private_Fields);

Latest Results (as of January 2022)

Time spent creating 100k instances that each initialize 100 fields.

Measured on Macbook Air (M1, 2020) 8GB running MacOS Monterey 12.1

Safari Tech Preview 137 includes this optimization

Field Initialization Technique	Safari Tech Preview 137 (Safari 15.4, WebKit 17613.1.11.8)	Chrome Canary 99.0.4828.0 arm64	Firefox Nightly 98.0a1 (2022-01-17) 64-bit
Classic Constructor Assignments	29ms	25ms	46ms
Public Fields	28ms	16ms	47ms
Private Fields	28ms	14ms	55ms

All significant performance issues are now resolved 🎉