Run results for: https://stackoverflow.com/posts/61510307

Run details:

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.122 Safari/537.36

Browser: Chrome 81

Operating system: Windows

Device Platform: Desktop

Date tested: 4 years ago

Test name	Executions per second
generator	2.2 Ops/sec
forloop object assign	2.3 Ops/sec
mapReduceSet	1.6 Ops/sec
forloopEntries	2.0 Ops/sec

Script Preparation code:

var nrows = 500000,
    nyears = [...Array(20).keys()].map(x => 2000 + x),
    nages = [...Array(20).keys()].map(x => `${18+x}`),
    npeople = [...Array(nrows / nyears.length / nages.length | 0).keys()].map(x => `Person${x}`);

var getOneRotate = (x, i) => x[(i / x.length | 0) % x.length];

var data = Array(nrows).fill(0).map((x, i) => ({
    year: getOneRotate(nyears,i),
    name: getOneRotate(npeople,i),
    age: getOneRotate(nages,i),
    value: (i / nyears.length | 0) % nyears.length
}));

const NA='NA';

function forloopObjectAssign(x) {
  let lname, lage, res=[], years = {};
for ( var i = 0 ; i < x.length ; i++ ) {
   const {year, value, name, age} = x[i];
   if ( lname != name || lage != age ) 
     res.push({name, age});
   res[ res.length-1 ][year] = value;
   
   years[year] = NA;
   lname = name;
   lage = age;
}
return res.map( x => Object.assign({...years}, x) );
}

function forloopEntries(x) {
let lname, lage, res=[], years = {};
for ( var i = 0 ; i < x.length ; i++ ) {
   const {year, value, name, age} = x[i];
   if ( lname != name|| lage != age ) 
     res.push([['name',name],['age',age]]);
   res[ res.length-1 ].push([year,value]);
   
   years[year] = NA;
   lname = name;
   lage = age;
}

return res.map( x => Object.fromEntries(Object.entries(years).concat(x)) );
}

function mapReduceSet(data) {
    years = new Set;
    result = Object
        .values(data.reduce((r, { name, year, value, ...o }) => {
            r[name] = r[name] || { name, ...o };
            r[name][year] = value;
            years.add(year);
            return r;
        }, {}))
        .map(
            (y => o => ({ ...y, ...o }))
            (Object.fromEntries([...years].map(y => [y, NaN])))
        );
    return result;
}

let yearsGenerator = {}
function* groupByName(entries) {
    let group = null;

    for (const {name, year, value, age} of entries) {
        if (group === null || group.name !== name || group.age !== age) {
            if (group !== null) {
                yield group;
            }
            
            group = {name, age};
        }

        group[year] = value;
        yearsGenerator[year] = 'NA';
    }

    if (group !== null) {
        yield group;
    }
}

function generator(entries) { return [...groupByName(entries)].map(x=>Object.assign({...yearsGenerator},x)) }

Tests:

generator
generator(data)
forloop object assign
forloopObjectAssign(data)
mapReduceSet
mapReduceSet(data)
forloopEntries
forloopEntries(data)

Run results for: https://stackoverflow.com/posts/61510307

Benchmark for transforming merged objects

generator

forloop object assign

mapReduceSet

forloopEntries