Run results for: Array spread operator vs push in reduce vs concat

Test name	Executions per second
spread operator	3485813.5 Ops/sec
Push	4132683.0 Ops/sec
Concat	2838651.5 Ops/sec

spread operator

var arr = [
        {
            "opportunityId": 5943111,
            "findingList": [
                {
                    "findingId": 6919
					 }
				],
		  },
		  {
				"opportunityId": 5943111,
            "findingList": [
                {
                    "findingId": 6918,
						   "findingId": 6917
					 }
				]
		  }
]

arr.reduce((acc, findingInOpp) => [...acc, ...findingInOpp.findingList.map(({ findingId }) => findingId)],[])

Push

var arr = [
        {
            "opportunityId": 5943111,
            "findingList": [
                {
                    "findingId": 6919
					 }
				],
		  },
		  {
				"opportunityId": 5943111,
            "findingList": [
                {
                    "findingId": 6918,
						   "findingId": 6917
					 }
				]
		  }
]

arr.reduce((acc, findingInOpp) => {
	acc.push(...findingInOpp.findingList.map(({ findingId }) => findingId))
	return acc
},[])

Concat

var arr = [
        {
            "opportunityId": 5943111,
            "findingList": [
                {
                    "findingId": 6919
					 }
				],
		  },
		  {
				"opportunityId": 5943111,
            "findingList": [
                {
                    "findingId": 6918,
						   "findingId": 6917
					 }
				]
		  }
]

arr.reduce((acc, findingInOpp) => acc.concat(findingInOpp.findingList.map(({ findingId }) => findingId)),[])

Run results for: Array spread operator vs push in reduce vs concat

Compare the new ES6 spread operator with push inside reduce vs concat

spread operator

Push

Concat