问题

I have just found that in ES6 there's a new math method: Math.trunc.

I have read its description in MDN article, and it sounds like using |0.

Moreover, <<0, >>0, &-1, ^0 also do similar things (thanks @kojiro & @Bergi).

After some tests, it seems that the only differences are:

Math.trunc returns -0 with numbers in interval (-1,-0]. Bitwise operators return 0.
Math.trunc returns NaN with non numbers. Bitwise operators return 0.

Are there more differences (among all of them)?

n      | Math.trunc | Bitwise operators
----------------------------------------
42.84  | 42         | 42
13.37  | 13         | 13
0.123  | 0          | 0
0      | 0          | 0
-0     | -0         | 0
-0.123 | -0         | 0
-42.84 | -42        | -42
NaN    | NaN        | 0
"foo"  | NaN        | 0
void(0)| NaN        | 0

回答1:

How about Math.trunc(Math.pow(2,31)) vs. Math.pow(2,31) | 0

Bitwise operations are performed on signed 32-bit integers. So, when you do Math.pow(2, 31) you get this representation in bits "10000000000000000000000000000000". Because this number has to be converted to signed 32-bit form, we now have a 1 in the sign bit position. This means that we are looking at a -eve number in signed 32-bit form. Then when we do the bitwise OR with 0 we get the same thing in signed 32-bit form. In decimal it is -2147483648.

Side note: In signed 32-bit form the range of decimals that can be represented in binary for is [10000000000000000000000000000000, 01111111111111111111111111111111]. In decimal (base 10) this range is [-2147483648, 2147483647].

回答2:

In many programming languages with bitwise operators, attempting to do a bitwise operation on a non-integer is a type error:

>>> # Python
>>> 1 << 0; 1.2 << 0
1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for <<: 'float' and 'int'

In ECMA-262, a Number is a double-precision 64-bit binary format IEEE 754. In other words, there are no integers in JavaScript. As long as the values you're dealing with fit within -(Math.pow(2,32)) and Math.pow(2,31) then the bitwise operations are a fast way to truncate floating point values. All of the different bitwise operations do different things, but in every example here they're essentially doing an identity operation. The critical difference is that JavaScript does a ToInt32 operation on the value before doing the nothing else part.

Bitwise identity operations:

i |  0 // For each bit that is 1, return 1|0. For each bit that is 0, return 0|0.
i ^  0 // xor, but effectively same as previous.
i << 0 // Shift the value left zero bits.
i >> 0 // Shift the value right zero bits.
i & -1 // Identity mask
~~i    // Not not - I had forgotten this one above.

来源：https://stackoverflow.com/questions/22156688/math-trunc-vs-0-vs-0-vs-0-vs-1-vs-0

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javascript