# Math.round()

`Math.round()` 函数返回一个数字四舍五入后最接近的整数。

## 语法

```Math.round(x)
```

`x`

## 示例

js
``````x = Math.round(20.49); //20
x = Math.round(20.5); //21
x = Math.round(-20.5); //-20
x = Math.round(-20.51); //-21
``````

### 小数舍入

js
``````// 闭包
(function () {
/**
* Decimal adjustment of a number.
*
* @param {String}  type  The type of adjustment.
* @param {Number}  value The number.
* @param {Integer} exp   The exponent (the 10 logarithm of the adjustment base).
* @returns {Number}      The adjusted value.
*/
// If the exp is undefined or zero...
if (typeof exp === "undefined" || +exp === 0) {
return Math[type](value);
}
value = +value;
exp = +exp;
// If the value is not a number or the exp is not an integer...
if (isNaN(value) || !(typeof exp === "number" && exp % 1 === 0)) {
return NaN;
}
// Shift
value = value.toString().split("e");
value = Math[type](+(value[0] + "e" + (value[1] ? +value[1] - exp : -exp)));
// Shift back
value = value.toString().split("e");
return +(value[0] + "e" + (value[1] ? +value[1] + exp : exp));
}

// Decimal round
if (!Math.round10) {
Math.round10 = function (value, exp) {
};
}
// Decimal floor
if (!Math.floor10) {
Math.floor10 = function (value, exp) {
};
}
// Decimal ceil
if (!Math.ceil10) {
Math.ceil10 = function (value, exp) {
};
}
})();

// Round
Math.round10(55.55, -1); // 55.6
Math.round10(55.549, -1); // 55.5
Math.round10(55, 1); // 60
Math.round10(54.9, 1); // 50
Math.round10(-55.55, -1); // -55.5
Math.round10(-55.551, -1); // -55.6
Math.round10(-55, 1); // -50
Math.round10(-55.1, 1); // -60
Math.round10(1.005, -2); // 1.01 -- compare this with Math.round(1.005*100)/100 above
// Floor
Math.floor10(55.59, -1); // 55.5
Math.floor10(59, 1); // 50
Math.floor10(-55.51, -1); // -55.6
Math.floor10(-51, 1); // -60
// Ceil
Math.ceil10(55.51, -1); // 55.6
Math.ceil10(51, 1); // 60
Math.ceil10(-55.59, -1); // -55.5
Math.ceil10(-59, 1); // -50
``````

js
``````function round(number, precision) {
return Math.round(+number + "e" + precision) / Math.pow(10, precision);
//same as:
//return Number(Math.round(+number + 'e' + precision) + 'e-' + precision);
}

round(1.005, 2); //1.01
``````

## 规范

Specification
ECMAScript Language Specification
# sec-math.round

## 浏览器兼容性

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