为什么 Node.js 会自动舍入我的浮点数？

Question

I'm trying to write a function that would fetch me the number of decimal places after the decimal point in a floating-point literal using the answer as a reference from here .

Although this seems to work fine when tried in the browser consoles, in the Node.js environment while running test cases, the precision is truncated only up to 14 digits.

let data = 123.834756380650877834678
console.log(data) // 123.83475638065088

And the function returns 14 as the answer.

Why is the rounding off happening at rest? Is it a default behavior?

Answer 1

The floating-point format used in JavaScript (and presumably Node.js) is IEEE-754 binary64. When 123.834756380650877834678 is used in source code, it is converted to the nearest representable value, which is 123.834756380650873097692965529859066009521484375.

When this is converted to a string with default formatting, JavaScript uses just enough digits to uniquely distinguish the value . For 123.834756380650873097692965529859066009521484375, this should produce “123.83475638065087”. If you are getting “123.83475638065088”, which differs in the last digit, then the software you are using does not conform to the JavaScript specification (ECMAScript).

In any case, the binary64 format does not have sufficient precision to preserve the information that the original numeral, “123.834756380650877834678”, has 21 digits after the decimal point.

The code you link to also does not and cannot compute the number of digits in an original numeral. It computes the number of digits needed to uniquely distinguish the value represented after conversion to binary64. For sufficiently short numerals without trailing zeros after the decimal point, this is the same as the number of digits after the decimal point in the original numeral. For others, it may not be.

Answer 2

it is default behavior of JavaScript.I think it will be same in node.js.

in JS The maximum number of decimals is 17.

for more details take a look at here