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
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.
来源:https://stackoverflow.com/questions/41679191/why-is-math-pow-sometimes-not-equal-to-in-javascript