JavaScript编号，内存大小相同？

Question

I'm reading the Number Type section of the book Professional JavaScript for Web Developers . It seems to say that all ECMAScript numbers are binary64 floating point, which is corroborated by this MDN article . But the book author also says:

Because storing floating-point values uses twice as much memory as storing integer values, ECMAScript always looks for ways to convert values into integers.

I expected numbers to each occupy the same amount of memory: 64 bits. And the MDN article says, "There is no specific type for integers". Anyone know what the book author meant? How can integers take up less memory when they're stored as 64-bit floats (if I have that right)? You'll find the whole section at the link above (free sample of the book).

Answer 1

JavaScript doesn't have any other number type than double precision floating point (except in ECMAScript 6 typed arrays ), but the underlying implementation may choose to store the numbers in any way it likes as long as the JavaScript code behaves the same.

JavaScript is compiled nowadays, which means that it can be optimised in many ways that are not obvious in the language.

If a local variable in a function only ever takes on an integer value and isn't exposed outside the function in any way, then it could actually be implemented using an integer type when the code is compiled.

The implementation varies in different browsers. Currently it seems to make a huge difference in MS Edge, a big difference in Firefox, and no difference at all in Chrome: http://jsperf.com/int-vs-double-implementation (Note: jsperf thinks that MS Edge is Chrome 42.)

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Further research:

The JS engines Spidermonkey (Firefox), V8 (Chrome, Opera), JavaScriptCore (Safari), Chakra (IE) and Rhino (and possibly others, but those are harder to find implementation details about) use different ways of using integer types or storing numbers as integers when possible. Some quotes:

"To have an efficient representation of numbers and JavaScript objects, V8 represents both of us with a 32 bits value. It uses a bit to know if it is an object (flag = 1) or an integer (flag = 0) called here SMall Integer or SMI because of its 31 bits."

http://thibaultlaurens.github.io/javascript/2013/04/29/how-the-v8-engine-works/

"JavaScript does not have a built-in notion of an integer value, but for efficiency JavaScriptCore will represent most integers as int32 rather than as double."

http://trac.webkit.org/wiki/JavaScriptCore

"[...] non-double values are a 32-bit type tag and a 32-bit payload, which is normally either a pointer or a signed 32-bit integer."

https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals

"In Windows 10 and Microsoft Edge, we've started optimizing Chakra's parser and the JIT compiler to identify non const variable declarations of integers that are defined globally and are never changed during the course of the execution time of the program."

https://blogs.windows.com/msedgedev/2015/05/20/delivering-fast-javascript-performance-in-microsoft-edge/

Answer 2

Because storing floating-point values uses twice as much memory as storing integer values, ECMAScript always looks for ways to convert values into integers.

This paragraph is complete nonsense. Ignore it!

Numbers are numbers. ECMAScript makes no distinction whatsoever between floating-point and integer numeric values.

Even within most JS runtimes, all numeric values are stored as double-precision floating point.

Answer 3

Not sure I fully understood your question, but "There is no specific type for integers" means that JavaScript doesn't recognize separate types for integers and floats, but they are both typed as Numbers. The int/float separation happens "behind the curtains", and that's what they meant by "ECMAScript always looks for ways to convert values into integers" .

The bottom line is that you don't have to worry about it, unless you specifically need your variables to mimic integers or floats for use in other languages, in which case it's probably (did I say probably ?) the best to pass them as strings (because you'd have trouble passing, say, 5.0 as a float because JS would immediatelly convert it to 5, exactly because of the "ECMAScript always looks for ways to convert values into integers" part).

 alert(5.0); // don't expect a float from this