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Floating-point cheat sheet for JavaScript
Tips for using floating-point and decimal numbers in JavaScript

Floating-Point Types

JavaScript is dynamically typed and will often convert implicitly between strings and floating-point numbers (which are IEEE 64 bit values). To force a variable to floating-point, use the global parseFloat() function.

	var num = parseFloat("3.5");

Decimal Types

The oldest decimal type for JavaScript seems to be a port of Java's BigDecimal class, which also supports rounding modes:

	var a = new BigDecimal("0.01");
	var b = new BigDecimal("0.02");
	var c = a.add(b); // 0.03
	var d = c.setScale(1, BigDecimal.prototype.ROUND_HALF_UP);

There is also bignumber.js, which is smaller and faster:

	BigNumber.config({ROUNDING_MODE: BigNumber.ROUND_HALF_UP})
	var a = new BigNumber("0.01");
	var b = new BigNumber("0.02");
	var c = a.plus(b); // BigNumber(0.03)
	var d = c.toFixed(1); // "0.0"

How to Round

	var num = 5.123456;
	num.toPrecision(1) //returns 5 as string
	num.toPrecision(2) //returns 5.1 as string
	num.toPrecision(4) //returns 5.123 as string

Using a specific rounding mode:

	new BigDecimal("1.25").setScale(1, BigDecimal.prototype.ROUND_HALF_UP);

Resources

title: 为 JavaScript 准备的浮点小抄 description: Tips for using floating-point and decimal numbers in JavaScript

浮点类型

JavaScript 是动态类型的，它经常暗地里在字符串和浮点数（IEEE 64 位值）之间转换。为了强制一个变量为浮点类型，可以使用全局函数 parseFloat()。

	var num = parseFloat("3.5");

小数类型

最古老的 JavaScript 小数类型是从 Java的 BigDecimal 类中移植的，它也支持 取整模式：

	var a = new BigDecimal("0.01");
	var b = new BigDecimal("0.02");
	var c = a.add(b); // 0.03
	var d = c.setScale(1, BigDecimal.prototype.ROUND_HALF_UP);

也有 bignumber.js，它更小更快：

	BigNumber.config({ROUNDING_MODE: BigNumber.ROUND_HALF_UP})
	var a = new BigNumber("0.01");
	var b = new BigNumber("0.02");
	var c = a.plus(b); // BigNumber(0.03)
	var d = c.toFixed(1); // "0.0"

如何取整

	var num = 5.123456;
	num.toPrecision(1) //以字符串形式返回 5 as string
	num.toPrecision(2) //以字符串形式返回 5.1 as string
	num.toPrecision(4) //以字符串形式返回 5.123 as string

使用特殊的取整模式：

	new BigDecimal("1.25").setScale(1, BigDecimal.prototype.ROUND_HALF_UP);

资源