SOURCE

function getUpperBound(num) {
    //如果是负数则先转正
    let isPositive = num > 0;
    //转科学计数
    let scientificNotation = num.toExponential();
    //截取前后两段备用
    let [a, b] = scientificNotation.split('e');
    //前面半段往5的倍数靠
    let upperBound = Math.ceil(Number(a *(isPositive?1:(-1)) / 5)) * 5 * (isPositive?1:(-1));
    //再乘回原来的量级
    return parseFloat(upperBound + 'e' + b);
}

console.log(getUpperBound(-703));