What is an alternating series? code example

Example: Alternating Sums

const alternatingSums = array => {
    let team1 = 0, team2 = 0
    array.forEach((num, index) => {
        if(index % 2 === 0) team1 += num
        else team2 += num
    })
    return [team1, team2]
}
/*
		// OR METHOD 2
*/
const alternatingSums = array => {
    let team1 = [], team2 = []
    array.forEach((num, index) => {
        if(index % 2 === 0) team1.push(num)
        else team2.push(num)
    })
    team1 = team1.reduce((a,b) => a + b, 0)
    team2 = team2.reduce((a,b) => a + b, 0)
    return [team1, team2]
}