Node.js / V8中没有真正的浮点类型？

Question

I try to store one float value through Buffer in Node.js

> f = 3.3
3.3
> var buf = new Buffer(32)
> buf.writeFloatBE(f);
4
> g = buf.readFloatBE();
3.299999952316284

Then I found the stored value g after readFloatBE() is NOT equal to the original f .

After further investigation, those two buffer values stored g and f are same.

> var buf1 = new Buffer(4); buf1.writeFloatBE(f); buf1
<Buffer 40 53 33 33>
> var buf2 = new Buffer(4); buf2.writeFloatBE(g); buf2
<Buffer 40 53 33 33>

According to this Buffer reading and writing floats , we know the writeDoulbeBE should be used here.

> var buf3 = new Buffer(8);
> buf3.writeDoubleBE(f);
8
> h = buf3.readDoubleBE();
3.3
> h === f
true

I want to know why the float type is not used in Node.js or V8 ? Refer to the code from V8

  // Fast primitive setters
  V8_INLINE void Set(bool value);
  V8_INLINE void Set(double i);
  V8_INLINE void Set(int32_t i);
  V8_INLINE void Set(uint32_t i);

It seems there is NO float type in V8 , any reason of this design or am I missing something? In which case, should this function writeFloatBE() be used?

Answer 1

It seems there is NO float type in V8

Yes, and this is by design: There is no float type in JavaScript either. All numbers are double s as specified by the ECMAScript standard .

Your number f is therefore 3.3 with double precision, while g only has float precision. As you can see, that's not the same double as f . The same would happen if you used one of the buf.writeInt… methods, the result after reading it would only be 3 not 3.3 .

In which case, should the function writeFloatBE() be used?

Yes, of course it should be used, whenever you want to store numbers with float precision in a buffer. If you want to store f with full precision, use writeDoubleBE instead.