编辑代码

function numFormat(num){
    num = num.toString().split('.');
    let arr = num[0].split('').reverse();
    let res = [];
    for(let i = 0; i < arr.length; ++i){      
        if(i%3 === 0 && i !== 0){
            res.push(',')
        }
        res.push(arr[i])
    }
    res = res.reverse().join('');
    if(num[1]){
        res = res.concat('.' + num[1])
    }

    return res;
}

var a=1234567894532;
var b=673439.4542;
console.log(numFormat(a)); // "1,234,567,894,532"
console.log(numFormat(b)); // "673,439.4542"