为什么JavaScript中的`btoa`编码适用于20位数字的字符串而不适用于20位数字的int? [英] Why does `btoa` encoding in javascript works for 20 digit string and not 20 digit int?

问题描述

考虑以下btoa输出

btoa(99999999999999999999)
"MTAwMDAwMDAwMDAwMDAwMDAwMDAw"

btoa(99999999999999999998)
"MTAwMDAwMDAwMDAwMDAwMDAwMDAw"

btoa("99999999999999999998")
"OTk5OTk5OTk5OTk5OTk5OTk5OTg="

btoa("99999999999999999999")
"OTk5OTk5OTk5OTk5OTk5OTk5OTk="

我们看到btoa无法编码20位int的唯一哈希，但是能够编码20位string.为什么会这样?

We see that btoa is unable to encode unique hash for 20 digit int but was able to encode 20 digit string. Why is this?

我最初的猜测是，由于btoa是基数64，因此它只能编码小于基数64的内容，但是为什么它却能够编码20位的数字string?

My original guess is that since btoa is base 64 it can only encode something that is less than base 64, but why is it capable of encoding a 20 digit string instead?

此外，btoa似乎无法编码小于9223372036854775807的int

Moreover btoa seems to not able to encode int less than 9223372036854775807 which is a 2^63

btoa(9223372036854775302)
"OTIyMzM3MjAzNjg1NDc3NjAwMA=="

btoa(9223372036854775303)
"OTIyMzM3MjAzNjg1NDc3NjAwMA=="

推荐答案

由于浮点数不精确.请记住，JavaScript在某些计算过程中只是暂时没有 integers ；所有JavaScript数字都是IEEE-754双精度浮点数. 99999999999999999999在IEEE-754数字中不是一个安全的整数值，实际上，如果您这样做:

Because of floating point imprecision. Remember, JavaScript doesn't have integers except temporarily during certain calculations; all JavaScript numbers are IEEE-754 double-precision floating point. 99999999999999999999 is not a safe integer value in IEEE-754 numbers, and in fact if you do this:

console.log(99999999999999999999);

...你会看到的

100000000000000000000

最大安全整数(例如，不受浮点不精确度影响的整数)为9007199254740991.

The max safe integer (e.g., integer that won't be affected by floating point imprecision) is 9007199254740991.

由于btoa接受 string (当您将数字传递给数字时，数字只会转换为字符串)，因此只需在值周围加上引号即可:

Since btoa accepts a string (when you pass it a number, the number just gets converted to string), just put quotes around your value:

btoa("99999999999999999999")
=> OTk5OTk5OTk5OTk5OTk5OTk5OTk=

当然，如果该值是数学计算的结果，则不能这样做.您必须更改计算如此大的数字的原因，因为它们超出了数字类型的精确范围.

Of course, if the value is the result of a math calculation, you can't do that. You'll have to change whatever it is that's calculating such large numbers, as they exceed the precise range of the number type.

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