Why "~undefined" is

 

This question already has an answer here:

  • What does a tilde do when it precedes an expression? 6 answers

    • ~ is bitwise NOT. It uses ToInt32 to convert the argument to a number. ToInt32 is defined as:

  • Let number be ToNumber(argument).
  • ReturnIfAbrupt(number).
  • If number is NaN, +0, −0, +∞, or −∞, return +0.
    ...

    • In turn, ToNumber(undefined) returns NaN , so according to step 3, ToInt32 returns 0 .

    And ~0 is -1 .

    every thing that cannot be represented in bits in JS for example "undefined, NaN" is treated a 0 or 0000000000000b for the ~ operator since it converts the operand into an signed integer see @felixkling answer for more details on this and since the operation ~ is BITwise not or 1s complement which flips the bits so the statement results in 111111111111b as a sequence of 1 and when dealing in numbers on binary level the MSB(most significant bit) is treated as a sign so when converting all the 0s to 1s it results in decimal value of -1 try ~0 for instance. and use this code to get the binary representation of a number (-3 >>> 0).toString(2))

    Apparently the bit representation of undefined is all 0 s. This can be seen if by the following: undefined | 0 undefined | 0 which evaluates to 0 . Because of this we know that undefined 's bit representation is all zeros.

    If we now filp all bits (wich is exactly what ~ does) we get all 1 s which is the representation of -1 .

    All this works because of javascript's type cohersion

    链接地址: http://www.djcxy.com/p/75040.html

    上一篇: JavaScript中的“！〜”是什么意思？

    下一篇: 为什么“〜undefined”是