符号>>在JavaScript中是什么意思

Question

What does the >> symbol mean? On this page , there's a line that looks like this:

var i = 0, l = this.length >> 0, curr;

Answer 1

It's bitwise shifting.

Let's take the number 7, which in binary is 0b00000111

7 << 1 shifts it one bit to the left, giving you 0b00001110 , which is 14

Similarly, you can shift to the right: 7 >> 1 will cut off the last bit, giving you 0b00000011 which is 3.

[Edit]
In JavaScript, numbers are stored as floats . However, when shifting you need integer values, so using bit shifting on JavaScript values will convert it from float to integer.

In JavaScript, shifting by 0 bits will round the number down* (integer rounding) (Better phrased: it will convert the value to integer)

> a = 7.5;
7.5
> a >> 0
7

*: Unless the number is negative.

Sidenote: since JavaScript's integers are 32-bit, avoid using bitwise shifts unless you're absolutely sure that you're not going to use large numbers.

[Edit 2]
this.length >> 0 will also make a copy of the number, instead of taking a reference to it. Although I have no idea why anyone would want that.

Answer 2

就像在许多其他语言中一样， >>运算符（在<<和>>> ）是一个按位移位 。