JS toFixed()填坑

问题

1、在JS中四舍五入的函数 toFixed(n) , n为要保留的小数位数。 n为0~20，当n超过20的时候，JS会出错。

2、当截取小数点前一位是0时，输出结果可能会有所不同。

var a = 0.145;

a.toFixed(2);

//结果为0.14；

靠谱分析

学会查标准
摘自ECMA-262 5.1 Edition

15.7.4.5 Number.prototype.toFixed (fractionDigits)

Return a String containing this Number value represented in decimal fixed-point notation with fractionDigits 
digits after the decimal point. If fractionDigits is undefined, 0 is assumed. Specifically, perform the following 
steps: 
1. Let f be ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the value 0). 
2. If f < 0 or f > 20, throw a RangeError exception. 
3. Let x be this Number value. 
4. If x is NaN, return the String "NaN". 
5. Let s be the empty String. 
6. If x < 0, then 
a. Let s be "-". 
b. Let x = –x. 
7. If x >= 10^21, then 
a. Let m = ToString(x). 
8. Else, x < 10^21
a. Let n be an integer for which the exact mathematical value of n / 10^f – x is as close to zero as possible. If there are two such n, pick the larger n. 
b. If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes). 
c. If f != 0, then 
i. Let k be the number of characters in m. 
ii. If k ≤ f, then 
1. Let z be the String consisting of f+1–k occurrences of the character '0'. 
2. Let m be the concatenation of Strings z and m. 
3. Let k = f + 1. 
iii. Let a be the first k–f characters of m, and let b be the remaining f characters of m. 
iv. Let m be the concatenation of the three Strings a, ".", and b. 
9. Return the concatenation of the Strings s and m. 

根据上述步骤

9.955.toFixed(2)的过程为

1. f = 2
3. x = 9.955
5. s = ''
8.

a. n = 996
原因
当 n = 995 时 n / 10^f – x = -0.005000000000000782
当 n = 996 时 n / 10^f – x = 0.005000000000000782
同样靠近0, 选择大的那个
b. m = '996'
c.
i. k = 3
iii. a = '9', b = '96'
iv. m = a + '.' + b = '9.96'
9. Return s + m = '9.96'

9.655.toFixed(2)的过程为

1. f = 2
3. x = 9.655
5. s = ''
8.

a. n = 965
原因
当 n = 965 时 n / 10^f – x = -0.004999999999999005
当 n = 966 时 n / 10^f – x = 0.005000000000000782
965更靠近0
b. m = '965'
c.
i. k = 3
iii. a = '9', b = '65'
iv. m = a + '.' + b = '9.65'
9. Return s + m = '9.65'

简而言之其实还是浮点误差的锅

修复

function toFixed(number, decimal) {
    decimal = decimal || 0;
    var s = String(number);
    var decimalIndex = s.indexOf('.');
    if (decimalIndex < 0) {
        var fraction = '';
        for (var i = 0; i < decimal; i++) {
            fraction += '0';
        }
        return s + '.' + fraction;
    }
    var numDigits = s.length - 1 - decimalIndex;
    if (numDigits <= decimal) {
        var fraction = '';
        for (var i = 0; i < decimal - numDigits; i++) {
            fraction += '0';
        }
        return s + fraction;
    }
    var digits = s.split('');
    var pos = decimalIndex + decimal;
    var roundDigit = digits[pos + 1];
    if (roundDigit > 4) {
        //跳过小数点
        if (pos == decimalIndex) {
            --pos;
        }
        digits[pos] = Number(digits[pos] || 0) + 1;
        //循环进位
        while (digits[pos] == 10) {
            digits[pos] = 0;
            --pos;
            if (pos == decimalIndex) {
                --pos;
            }
            digits[pos] = Number(digits[pos] || 0) + 1;
        }
    }
    //避免包含末尾的.符号
    if (decimal == 0) {
        decimal--;
    }
    return digits.slice(0, decimalIndex + decimal + 1).join('');
}

实验：

toFixed(0.0056,2)
//'0.01'
toFixed(298561.7249999999999999,2)
//'298561.73'
toFixed(298561.7249999999999999,3)
//'298561.725'