var x = 0.3445434
x = Math.round (x*100) / 100 // this will make nice rounding

countDecimals = value => { if (Math.floor(value) === value) return 0; let stringValue = value.toString().split(".")[1]; if (stringValue) { return value.toString().split(".")[1].length ? value.toString().split(".")[1].length : 0; } else { return 0; } }; formatNumber=(ans)=>{ let decimalPlaces = this.countDecimals(ans); ans = 1 * ans; if (decimalPlaces !== 0) { let onePlusAns = ans + 1; let decimalOnePlus = this.countDecimals(onePlusAns); if (decimalOnePlus < decimalPlaces) { ans = ans.toFixed(decimalPlaces - 1).replace(/\.?0*$/, ""); } else { let tenMulAns = ans * 10; let decimalTenMul = this.countDecimals(tenMulAns); if (decimalTenMul + 1 < decimalPlaces) { ans = ans.toFixed(decimalPlaces - 1).replace(/\.?0*$/, ""); } } } }

我只是给这个值加上1，然后数出原始值和增加的值中出现的十进制数字。如果我在比原始十进制数字少加1之后找到十进制数字，我只调用toFixed() with(原始小数- 1)。我还通过将原始值乘以10来检查，并遵循相同的逻辑，以防加1不会减少冗余小数位。 在JS中处理浮点数舍入的简单变通方法。我试过的大多数情况下都有效。

/** don't spend 5 minutes, use my code **/
function prettyFloat(x,nbDec) { 
    if (!nbDec) nbDec = 100;
    var a = Math.abs(x);
    var e = Math.floor(a);
    var d = Math.round((a-e)*nbDec); if (d == nbDec) { d=0; e++; }
    var signStr = (x<0) ? "-" : " ";
    var decStr = d.toString(); var tmp = 10; while(tmp<nbDec && d*tmp < nbDec) {decStr = "0"+decStr; tmp*=10;}
    var eStr = e.toString();
    return signStr+eStr+"."+decStr;
}

prettyFloat(0);      //  "0.00"
prettyFloat(-1);     // "-1.00"
prettyFloat(-0.999); // "-1.00"
prettyFloat(0.5);    //  "0.50"

There is no way to avoid inconsistent rounding for prices with x.xx5 as actual value using either multiplication or division. If you need to calculate correct prices client-side you should keep all amounts in cents. This is due to the nature of the internal representation of numeric values in JavaScript. Notice that Excel suffers from the same problems so most people wouldn't notice the small errors caused by this phenomen. However errors may accumulate whenever you add up a lot of calculated values, there is a whole theory around this involving the order of calculations and other methods to minimize the error in the final result. To emphasize on the problems with decimal values, please note that 0.1 + 0.2 is not exactly equal to 0.3 in JavaScript, while 1 + 2 is equal to 3.

使用乘法器的所有解都存在一个问题。不幸的是，kkyy和Christoph的解决方案都是错误的。

请测试数字551.175的小数点后2位-它将四舍五入为551.17，而它应该是551.18 !但是如果你测试的是451.175，就可以了- 451.18。所以第一眼很难发现这个错误。

问题在于乘法:尝试551.175 * 100 = 55117.49999999999 (up !)

所以我的想法是在使用Math.round()之前用toFixed()处理它;

function roundFix(number, precision)
{
    var multi = Math.pow(10, precision);
    return Math.round( (number * multi).toFixed(precision + 1) ) / multi;
}

我使用这段代码来格式化浮动。它基于toPrecision()，但它去掉了不必要的零。我欢迎关于如何简化正则表达式的建议。

function round(x, n) {
    var exp = Math.pow(10, n);
    return Math.floor(x*exp + 0.5)/exp;
}

使用的例子:

function test(x, n, d) {
    var rounded = rnd(x, d);
    var result = rounded.toPrecision(n);
    result = result.replace(/\.?0*$/, '');
    result = result.replace(/\.?0*e/, 'e');
    result = result.replace('e+', 'e');
    return result;  
}

document.write(test(1.2000e45, 3, 2) + '=' + '1.2e45' + '<br>');
document.write(test(1.2000e+45, 3, 2) + '=' + '1.2e45' + '<br>');
document.write(test(1.2340e45, 3, 2) + '=' + '1.23e45' + '<br>');
document.write(test(1.2350e45, 3, 2) + '=' + '1.24e45' + '<br>');
document.write(test(1.0000, 3, 2) + '=' + '1' + '<br>');
document.write(test(1.0100, 3, 2) + '=' + '1.01' + '<br>');
document.write(test(1.2340, 4, 2) + '=' + '1.23' + '<br>');
document.write(test(1.2350, 4, 2) + '=' + '1.24' + '<br>');