js保留小数点后n位，不四舍五入

function toFixedDigit(num,n){
    
    //保留n位
    if(typeof num != 'number'){
    
    
        return false;
    };
    
    num = num.toString();
    var result = "";
    var zeroResult = function(n){
    
    
        var zero = "";
        for(var i = 0;i<n;i++){
    
    
            zero+="0";
        }
        return zero;
    }
    if(num%1 == 0){
    
    //整数
        result = num + "."+zeroResult(n);
    }else{
    
    //小数
        var num1 = num.split(".");
        if(num1[1].length<n){
    
    
           result = num1[0]+"."+num1[1]+zeroResult(n-num1[1].length)
        }else{
    
    
           result = num1[0]+"."+num1[1].substring(0,n)
        }
    }
    return result;
}
var result1 = toFixedDigit(8.7,2)
console.log(result1)//8.70
var result2 = toFixedDigit(8,2)
console.log(result2)//8.00
var result3 = toFixedDigit(8.6999,2)
console.log(result3)//8.69
var result4 = toFixedDigit(85.6999765,4)
console.log(result4)//85.6999