将 8 字节 (u64) 无符号整数转换为 javascript 的建议

Question

I have a u64 (unsigned integer) stored in 8 bytes of memory. Clearly the range is 0-2^64 integers.

I am converting it to a javascript number by turning each byte into hex and making a hex string:

let s = '0x'
s += buffer.slice(0,1).toString("hex")
s += buffer.slice(1,2).toString("hex")
...
n = parseInt(s)

Works great for everything I have done so far.

But when I look at how javascript stores numbers, I become unsure. Javascript uses 8 bytes for numbers, but treats all numbers the same. This internal javascript "number" representation can also hold floating point numbers.

Can a javascript number store all integers from 0 to 2^64? seems not.

At what point do I get into trouble? What do people do to get round this?

Answer 1

An unsigned 64 bit integer has the range of a 0 to 18.446.744.073.709.551.615.

You could use the Number wrapper object with the .MAX_VALUE property, it represents the maximum numeric value representable in JavaScript .

The JavaScript Number type is a double-precision 64-bit binary format IEEE 754 value, like double in Java or C#.

General Info:

Integers in JS:

JavaScript has only floating-point numbers. Integers appear internally in two ways. First, most JavaScript engines store a small enough number without a decimal fraction as an integer (with, for example, 31 bits) and maintain that representation as long as possible. They have to switch back to a floating point representation if a number's magnitude grows too large or if a decimal fraction appears. Second, the ECMAScript specification has integer operators: namely, all of the bitwise operators. Those operators convert their operands to 32-bit integers and return 32-bit integers. For the specification, integer only means that the numbers don't have a decimal fraction, and 32-bit means that they are within a certain range. For engines, 32-bit integer means that an actual integer (non-floating-point) representation can usually be introduced or maintained.

Ranges of integers

Internally, the following ranges of integers are important in JavaScript:

• Safe integers [1], the largest practically usable range of integers that JavaScript supports: 53 bits plus a sign, range (−2^53, 2^53) which relates to (+/-) 9.007.199.254.740.992
• Array indices [2]: 32 bits, unsigned Maximum length: 2^32−1 Range of indices: [0, 2^32−1) (excluding the maximum length!)
• Bitwise operands [3]: Unsigned right shift operator (>>>): 32 bits, unsigned, range [0, 2^32) All other bitwise operators: 32 bits, including a sign, range [−2^31, 2^31)
• “Char codes”, UTF-16 code units as numbers: Accepted by String.fromCharCode() Returned by String.prototype.charCodeAt() 16 bit, unsigned

References:

[1] Safe integers in JavaScript

[2] Arrays in JavaScript

[3] Label bitwise_ops

Source: https://2ality.com/2014/02/javascript-integers.html