为什么Math.pow（）（有时）不等于JavaScript中的**？ [英] Why is Math.pow() (sometimes) not equal to ** in JavaScript?

问题描述

我刚刚发现了ECMAScript 7特性 a ** b 作为 Math.pow（a，b）（ MDN参考资料），并在中发现该帖子，其中他们显然表现不同。我已经在Chrome 55中进行了测试，并且可以确认结果不同。

Math.pow（99,99）返回 3.697296376497263e + 197

而

99 ** 99 返回 3.697296376497268e + 197

因此，记录差异 Math.pow（99,99） - 99 ** 99 结果 -5.311379928167671e + 182 。

到目前为止，可以这么说，它只是另一个实现，但是将它封装到一个函数中会有不同的表现：

 函数diff（x）{
 return Math.pow（x，x） -  x ** x; 
 
  

调用 diff（99）返回 0 。

为什么会这样？

正如 xszaboj 所指出的那样，这可以归结为这个问题：

  var x = 99; 
 x ** x  -  99 ** 99; //返回-5.311379928167671e + 182 
  

解决方案

99 ** 99 在编译时评估为时间（常量折叠）和编译器的 pow 例程不同于运行时间。在运行时评估 ** 时，结果与 Math.pow 相同 - 难怪因为 ** 实际上是编译到 Math.pow 调用：

 的console.log（99 ** 99）; // 3.697296376497268e + 197a = 99，b = 99; console.log（a ** b）; // 3.697296376497263e + 197console.log（Math.pow（99，99））; // 3.697296376497263e + 197  

其实$
$ b

^{99 99 = 369729637649726772657187905628805440595668764281741102430259972423552570455277523421410650010128232727940978889548326540119429996769494359451621570193644014418071060667659301384999779999159200499899}

这样的第一个结果是一种更好的近似，不应该发生常量表达式和动态表达式之间的这种差异。

这种行为看起来像V8中的一个错误。它已被举报，并希望很快得到解决。

I've just discovered the ECMAScript 7 feature a**b as an alternative for Math.pow(a,b) (MDN Reference) and came across a discussion in that post, in which they apparently behave differently. I've tested it in Chrome 55 and can confirm that the results differ.

Math.pow(99,99) returns 3.697296376497263e+197

whereas

99**99 returns 3.697296376497268e+197

So logging the difference Math.pow(99,99) - 99**99 results in -5.311379928167671e+182.

So far it could be said, that it's simply another implementation, but wrapping it in a function behaves different again:

function diff(x) {
  return Math.pow(x,x) - x**x;
}

calling diff(99) returns 0.

Why is that happening?

As xszaboj pointed out, this can be narrowed down to this problem:

var x = 99;
x**x - 99**99; // Returns -5.311379928167671e+182

解决方案

99**99 is evaluated at compile time ("constant folding"), and the compiler's pow routine is different from the runtime one. When evaluating ** at run time, results are identical with Math.pow — no wonder since ** is actually compiled to a Math.pow call:

console.log(99**99);           // 3.697296376497268e+197
a = 99, b = 99;
console.log(a**b);             // 3.697296376497263e+197
console.log(Math.pow(99, 99)); // 3.697296376497263e+197

Actually

^{9999=369729637649726772657187905628805440595668764281741102430259972423552570455277523421410650010128232727940978889548326540119429996769494359451621570193644014418071060667659301384999779999159200499899}

so the first result is a better approximation, still such a discrepancy between constant- and dynamic expressions shouldn't take place.

This behavior looks like a bug in V8. It has been reported and will hopefully get fixed soon.

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