scales = [{unit: 'g', scale: 1}, {unit: 'dag', scale: 10}, {unit: 'hg', scale: 100}, {unit: 'kg', scale: 1000}, {unit: 'mg', scale: 0.001}, {unit: 'cg', scale: 0.01}, {unit: 'dg', scale: 0.1}]

function bestNotation(num_g, min_scale, max_scale) {
    var exp = 0
    var result = num_g
    while(result >= 10 && 10**exp < max_scale) {
        result /= 10
        exp++
    }
    while(result < 1 && 10**exp > min_scale) {
        result *= 10
        exp--
    }

    unit_object = scales.find(x => x.scale === 10**exp)
    return result + ' ' + unit_object.unit
}

// Risultati testati
// bestNotation(1284, 0.001, 1000) -> "1.284 kg"
// bestNotation(0.052, 0.001, 1000) -> "5.2 cg"
// bestNotation(250000, 0.001, 1000) -> "250 kg"
// bestNotation(0.000000001, 0.001, 1000) -> "0.000001 mg"
// bestNotation(5.3, 0.001, 1000) -> "5.3 g"