JavaScript - 为什么（0 || -0）返回-0

Question

I've tested the following code 0 || -0 0 || -0 in the Chrome Developer Console and I've got back -0 as a result and I'm trying to figure out why. This table represents results in different JS environments.

╔══════════════════════════════════════════════════════╗
║ Browser                  ║ Version         ║ 0 || -0 ║
╠══════════════════════════════════════════════════════╣
║Internet Explorer 11      ║ 11.3024.14393.0 ║    0    ║
║Google Chrome             ║ 75.0.3770.100   ║   -0    ║
║Mozilla Firefox (Quantum) ║ 67.0.4          ║   -0    ║
║Microsoft Edge (Chromium) ║ 77.0.197.1      ║   -0    ║
║Microsoft Edge (Chakra)   ║ 44.17763.1.0    ║    0    ║
║Node.js                   ║ 11.1.0          ║   -0    ║
╚══════════════════════════════════════════════════════╝

My assumption is that in the first case, when the -0 is on the right side, the result is -0 , however when it's on the left side, the result is 0 . This leads me to believe that when -0 is on the left side during the binary logical OR operation, the sign bit is discarded (for whatever reason) and the operation is performed like this 0 || 0 0 || 0 . While, when it's on the right side, because the - operator is present, it probably added a sign bit to the number, which results in the right side number having one extra bit, causing the right side value to be returned.

What's even more interesting is that when you use positive numbers, the behavior is different. Now the -1 || 4 -1 || 4 returns -1 , while 4 || -1 4 || -1 returns 4 and 4 || -1 || 6 4 || -1 || 6 4 || -1 || 6 returns 4 (shouldn't it return 6 ). Is this behavior caused by the - operator or is it because I'm testing this code in the developer console?

Answer 1

Per the official specification, a || b a || b evaluates to a if a is truthy, or b if a is falsey. && works in a similar fashion - if the whole expression's value can be determined by just checking the value of the left-hand side, it will evaluate to the left-hand side, and will otherwise evaluate to the right-hand side.

See MDN :

Logical OR (||)

expr1 || expr2

If expr1 can be converted to true, returns expr1; else, returns expr2.

Because both 0 and -0 are falsey, 0 || -0 0 || -0 should evaluate to -0 , because the left side was falsey.

Comparing numbers other than 0 / -0 will have different effects, because other non-zero numbers are not falsey (except for NaN ).

Perhaps IE and Edge-Chakra are simply ignoring the difference between 0 and -0 , and display 0 regardless. For example, if you type -0 into IE's console, you get 0 as a result ( not -0 ):

Edge does the same thing:

The difference may just be purely visual, though. The following snippet prints true , false , in Edge, as expected:

 const x = -0; console.log(Object.is(x, -0)); console.log(Object.is(x, 0)); 

Answer 2

" binary logical OR " may be where the assumption gets problematic: there is bitwise/binary or, | , looking at the digits of the binary numbers, and logical or, || , looking at each operand as a single true/false value, and thus they are rather different.

You have the latter one, that is why you get back one of the exact inputs and not a "combination" of them. This is especially visible with -1 , as in two's component code it is 111...1111 ( 32 of them in case of JavaScript when bitwise operations are involved), and thus binary or-ing anything to it just reproduces -1 . But in case of a logical or, you get back the first operand unchanged, if it is 'true-ish', or you get back the second operand, again unchanged.

(Of course the MS thing with the -0 described in the other answer is adding some confusion for sure, but it happens before, and independently from the logical operation)