const arr2obj = (arr, reverse = false, isValid = (x) => { return true }) => {
    let obj = {}
    
    arr.forEach((v, i) => {
        if (!(isValid(i))) return      
        reverse ? obj[v] = i : obj[i] = v
    })
    
    return obj
}

a = Array.from('abcdefg')
console.log(arr2obj(a, true, (x) => { return x % 2 !== 0 }))