我想最多四舍五入两位小数，但只有在必要时。
输入：
10
1.7777777
9.1
输出：
10
1.78
9.1
如何在JavaScript中执行此操作？
当前回答
另一种方法是：
number = 16.6666666;
console.log(parseFloat(number.toFixed(2)));
"16.67"
number = 16.6;
console.log(parseFloat(number.toFixed(2)));
"16.6"
number = 16;
console.log(parseFloat(number.toFixed(2)));
"16"
.toFixed（2）返回一个正好有两个小数点的字符串，可以是尾随零，也可以不是尾随零。执行parseFloat（）将消除那些尾随的零。
其他回答
简单的通用舍入函数如下：
步骤如下：
使用Math.pow（10，位）将数字乘以（10乘以小数位数的幂）。使用Math.Round将结果舍入为整数。将结果除以（10乘以小数位数的幂）Math.pow（10，位）。
例子：
数字为：1.2375四舍五入至小数点后三位
1.2375 * (10^3) ==> 1.2375 * 1000 = 1237.5舍入为整数==>1238将1238除以（10^3）==>1238/1000=1.238
（注：10^3表示数学功率（10,3））。
函数编号RoundDecimal（v，n）{return Math.round（（v+Number.EPSILON）*Math.pow（10，n））/Math.pow（1，n）}//-------测试--------console.log（numberRoundDecimal（-0.0246411603862896567,3））//-0.025console.log（numberRoundDecimal（0.9993360575508052,3））//0.999console.log（numberRoundDecimal（1.0020739645577939，3））//1.002console.log（numberRoundDecimal（0.975,0））//1console.log（numberRoundDecimal（0.975,1））//1console.log（numberRoundDecimal（0.975,2））//0.98console.log（numberRoundDecimal（1.005,2））//1.01
如果您需要将货币金额格式化为整数货币或包含小数货币部分的金额，则会有一点不同。
例如：
1应输出$1
1.1应产出1.10美元
1.01应产出1.01美元
假设金额是一个数字：
常量格式amount=（amount）=>amount%1==0？amount：amount.toFixed（2）；
如果amount不是数字，则使用parseFloat（amount）将其转换为数字。
只有在必要的时候你说？
如果你也关心负数，我建议你。。。
有些答案对负数不太有效。。。
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>roundPrecision</title>
<script>
class MyMath{
static roundPrecision(number, precision, fillZeros) {
// Number you want to round
// precision nb of decimals
// fillZeros the number of 0 You want to add IF necessary!
// 0 = no fill with zeros.
let num = number;
let prec = precision;
let exp = Math.pow(10, prec);
let round = Math.round(number * exp)/exp
if (fillZeros>0) {
return round.toFixed(fillZeros)
}
return round;
}
}
</script>
</head>
<body>
<p class="myMath" id="field1"></p>
<p class="myMath" id="field2"></p>
<p class="myMath" id="field3"></p>
<p class="myMath" id="field4"></p>
<p class="myMath" id="field5"></p>
<p class="myMath" id="field6"></p>
<p class="myMath" id="field7"></p>
<script>
document.getElementById("field1").innerHTML = MyMath.roundPrecision(5, 0, 3); // 5.000
document.getElementById("field2").innerHTML = MyMath.roundPrecision(Math.PI, 2, 4); // 3.1400
document.getElementById("field3").innerHTML = MyMath.roundPrecision(2.4, 1, 2); // 2.40
document.getElementById("field4").innerHTML = MyMath.roundPrecision(2.9, 0, 2); // 3.00
document.getElementById("field5").innerHTML = MyMath.roundPrecision(10, 0, 2); // 10.00
document.getElementById("field6").innerHTML = MyMath.roundPrecision(-10.5, 1, 2); // 10.00
document.getElementById("field7").innerHTML = MyMath.roundPrecision(-1.006, 2, 0); // 10.00
</script>
</body>
</html>
我的解决方案将输入视为字符串，并使用n位数字的“数学舍入”算法：取n位数字，如果数字n+1等于或大于5，则加1。它还允许指定负数，例如，将123.45舍入为-1的数字为120。它也适用于科学符号（例如1.2e-3）。我没有测量它的速度，我认为这不是最好的表现。
function safeRound( numInput, numPrecision ) {
const strNumber = numInput.toString().replace( 'E', 'e' );
const bSign = '+-'.indexOf( strNumber[ 0 ] ) !== -1;
const strSign = bSign ? strNumber[ 0 ] : '';
const numSign = strSign !== '-' ? +1 : -1;
const ixExponent = ( ixFound => ixFound !== -1 ? ixFound : strNumber.length )( strNumber.indexOf( 'e' ) );
const strExponent = strNumber.substr( ixExponent + 1 );
const numExponent = ixExponent !== strNumber.length ? Number.parseInt( strExponent ) : 0;
const ixDecimal = ( ixFound => ixFound !== -1 ? ixFound : ixExponent )( strNumber.indexOf( '.' ) );
const strInteger = strNumber.substring( !bSign ? 0 : 1, ixDecimal );
const strFraction = strNumber.substring( ixDecimal + 1, ixExponent );
const numPrecisionAdjusted = numPrecision + numExponent;
const strIntegerKeep = strInteger.substring( 0, strInteger.length + Math.min( 0, numPrecisionAdjusted ) ) + '0'.repeat( -Math.min( 0, numPrecisionAdjusted ) );
const strFractionKeep = strFraction.substring( 0, Math.max( 0, numPrecisionAdjusted ) );
const strRoundedDown = strSign + ( strIntegerKeep === '' ? '0' : strIntegerKeep ) + ( strFractionKeep === '' ? '' : '.' + strFractionKeep ) + ( strExponent === '' ? '' : 'e' + strExponent );
const chRoundUp = 0 <= numPrecisionAdjusted ? strFraction.substr( numPrecisionAdjusted, 1 ) : ( '0' + strInteger ).substr( numPrecisionAdjusted, 1 );
const bRoundUp = '5' <= chRoundUp && chRoundUp <= '9';
const numRoundUp = bRoundUp ? numSign * Math.pow( 10, -numPrecision ) : 0;
return Number.parseFloat( strRoundedDown ) + numRoundUp;
}
function safeRoundTest( numInput, numPrecision, strExpected ) {
const strActual = safeRound( numInput, numPrecision ).toString();
const bPassed = strActual === strExpected;
console.log( 'numInput', numInput, 'numPrecision', numPrecision, 'strExpected', strExpected, 'strActual', strActual, 'bPassed', bPassed );
return bPassed ? 0 : 1;
}
function safeRoundTests() {
let numFailed = 0;
numFailed += safeRoundTest( 0, 0, '0' );
numFailed += safeRoundTest( '0', 0, '0' );
numFailed += safeRoundTest( '0.1', 0, '0' );
numFailed += safeRoundTest( '+0.1', 0, '0' );
numFailed += safeRoundTest( '-0.1', 0, '0' );
numFailed += safeRoundTest( '0.1', 1, '0.1' );
numFailed += safeRoundTest( '+0.1', 1, '0.1' );
numFailed += safeRoundTest( '-0.1', 1, '-0.1' );
numFailed += safeRoundTest( '0.9', 0, '1' );
numFailed += safeRoundTest( '+0.9', 0, '1' );
numFailed += safeRoundTest( '-0.9', 0, '-1' );
numFailed += safeRoundTest( '0.9', 1, '0.9' );
numFailed += safeRoundTest( '+0.9', 1, '0.9' );
numFailed += safeRoundTest( '-0.9', 1, '-0.9' );
numFailed += safeRoundTest( '0.5', 0, '1' );
numFailed += safeRoundTest( '+0.5', 0, '1' );
numFailed += safeRoundTest( '-0.5', 0, '-1' );
numFailed += safeRoundTest( '0.4999', 0, '0' );
numFailed += safeRoundTest( '+0.4999', 0, '0' );
numFailed += safeRoundTest( '-0.4999', 0, '0' );
numFailed += safeRoundTest( '1.005', 2, '1.01' );
numFailed += safeRoundTest( '1.00499999999', 2, '1' );
numFailed += safeRoundTest( '012.3456', -4, '0' );
numFailed += safeRoundTest( '012.3456', -3, '0' );
numFailed += safeRoundTest( '012.3456', -2, '0' );
numFailed += safeRoundTest( '012.3456', -1, '10' );
numFailed += safeRoundTest( '012.3456', 0, '12' );
numFailed += safeRoundTest( '012.3456', 1, '12.3' );
numFailed += safeRoundTest( '012.3456', 2, '12.35' );
numFailed += safeRoundTest( '012.3456', 3, '12.346' );
numFailed += safeRoundTest( '012.3456', 4, '12.3456' );
numFailed += safeRoundTest( '012.3456', 5, '12.3456' );
numFailed += safeRoundTest( '12.', 0, '12' );
numFailed += safeRoundTest( '.12', 2, '0.12' );
numFailed += safeRoundTest( '0e0', 0, '0' );
numFailed += safeRoundTest( '1.2e3', 0, '1200' );
numFailed += safeRoundTest( '1.2e+3', 0, '1200' );
numFailed += safeRoundTest( '1.2e-3', 0, '0' );
numFailed += safeRoundTest( '1.2e-3', 3, '0.001' );
numFailed += safeRoundTest( '1.2e-3', 4, '0.0012' );
numFailed += safeRoundTest( '1.2e-3', 5, '0.0012' );
numFailed += safeRoundTest( '+12.', 0, '12' );
numFailed += safeRoundTest( '+.12', 2, '0.12' );
numFailed += safeRoundTest( '+0e0', 0, '0' );
numFailed += safeRoundTest( '+1.2e3', 0, '1200' );
numFailed += safeRoundTest( '+1.2e+3', 0, '1200' );
numFailed += safeRoundTest( '+1.2e-3', 0, '0' );
numFailed += safeRoundTest( '+1.2e-3', 3, '0.001' );
numFailed += safeRoundTest( '+1.2e-3', 4, '0.0012' );
numFailed += safeRoundTest( '+1.2e-3', 5, '0.0012' );
numFailed += safeRoundTest( '-12.', 0, '-12' );
numFailed += safeRoundTest( '-.12', 2, '-0.12' );
numFailed += safeRoundTest( '-0e0', 0, '0' );
numFailed += safeRoundTest( '-1.2e3', 0, '-1200' );
numFailed += safeRoundTest( '-1.2e+3', 0, '-1200' );
numFailed += safeRoundTest( '-1.2e-3', 0, '0' );
numFailed += safeRoundTest( '-1.2e-3', 3, '-0.001' );
numFailed += safeRoundTest( '-1.2e-3', 4, '-0.0012' );
numFailed += safeRoundTest( '-1.2e-3', 5, '-0.0012' );
numFailed += safeRoundTest( '9876.543e210', 0, '9.876543e+213' );
numFailed += safeRoundTest( '9876.543e210', -210, '9.877e+213' );
numFailed += safeRoundTest( '9876.543e210', -209, '9.8765e+213' );
numFailed += safeRoundTest( '9876.543e+210', 0, '9.876543e+213' );
numFailed += safeRoundTest( '9876.543e+210', -210, '9.877e+213' );
numFailed += safeRoundTest( '9876.543e+210', -209, '9.8765e+213' );
numFailed += safeRoundTest( '9876.543e-210', 213, '9.876543e-207' );
numFailed += safeRoundTest( '9876.543e-210', 210, '9.877e-207' );
numFailed += safeRoundTest( '9876.543e-210', 211, '9.8765e-207' );
console.log( 'numFailed', numFailed );
}
safeRoundTests();
parseFloat（“1.555”）.toFixed（2）；//返回1.55而不是1.56。
1.55是绝对正确的结果，因为在计算机中不存在1.555的精确表示。如果读数为1.555，则四舍五入至最接近的值=1.55499999999999994（64位浮点）。将这个数字四舍五入到Fixed（2）得到1.55。
如果输入为1.55499999999999，则此处提供的所有其他功能都会给出故障结果。
解决方案：在扫描前加上数字“5”，将数字舍入（更准确地说，从0开始舍入）。仅当数字真的是浮点（有小数点）时才执行此操作。
parseFloat("1.555"+"5").toFixed(2); // Returns 1.56
