javascript - 解释: Math. fround()函数

Question

Math.fround() 函数是如何工作的。

The Math.fround() function returns the nearest float representation of a number.

但是当它传递 float Math.fround(1.5) 时，它返回相同的 (1.5) 值。但是当它传递 float Math.fround(1.55) 时，它返回一个不同的值 1.5499999523162841。 为什么以及如何？

• I am confused about how Math.fround() works.
• How is it different from the Math.round() function?
• Also let me know why it's not supported in Internet Explorer.

Answer 1

要了解此功能的工作原理，您实际上必须了解以下主题:

• JavaScript’s Number type in details
• The simple math behind decimal-binary conversion algorithms
• How to round binary numbers
• The mechanics behind exponent bias in floating point

ECMA 脚本指定 the following algorithm对于转换:

When Math.fround is called with argument x, the following steps are taken:

• If x is NaN, return NaN.
• If x is one of +0, -0, +∞, -∞, return x.
• Let x32 be the result of converting x to a value in IEEE 754-2008 binary32 format using roundTiesToEven.
• Let x64 be the result of converting x32 to a value in IEEE 754-2008 binary64 format.
• Return the ECMAScript Number value corresponding to x64.

那么，让我们为 1.5 和 1.55 这样做。

数学.fround(1.5)

1)以64位 float 表示

0 01111111111 1000000000000000000000000000000000000000000000000000

2)用科学记数法表示

1.1000000000000000000000000000000000000000000000000000 x 2^0

3) 四舍五入到23位尾数

1.10000000000000000000000

4) 转换为十进制:

1.5

Math.fround(1.55)

1)以64位 float 表示

0 01111111111 1000110011001100110011001100110011001100110011001101

2)用科学记数法表示

1.1000110011001100110011001100110011001100110011001101 x 2^0

3) 四舍五入到23位尾数

1.10001100110011001100110

4) 转换为十进制:

1.5499999523162841