The page I am working on has a javascript function executed to print parts of the page. For some reason, printing in Safari, causes the window to somehow update. I say somehow, because it does not really refresh as in reload the page, but rather it starts the "rendering" of the page from start, i.e. scroll to top, flash animations start from 0, and so forth. The effect is reproduced by this fiddle: http://jsfiddle.net/fYmnB/ Clicking the print button and finishing or cancelling a print in Safari causes the screen to "go white" for a sec, which in my real website manifests itself as something "like" a reload. While running print button with, let's say, Firefox, just opens and closes the print dialogue without affecting the fiddle page in any way. Is there something with my way of calling the browsers print method that causes this, or how can it be explained - and preferably, avoided? P.S.: On my real site the same occurs with Chrome. In the ex
It removes everything after the decimal point because the bitwise operators implicitly convert their operands to signed 32-bit integers. This works whether the operands are (floating-point) numbers or strings, and the result is a number.
ReplyDeleteIn other words, it yields:
function(x) {
if(x < 0) return Math.ceil(x);
else return Math.floor(x);
}
only if x is between -(231) and 231 - 1. Otherwise, overflow will occur and the number will "wrap around".
This may be considered useful to convert a function's string argument to a number, but both because of the possibility of overflow and that it is incorrect for use with non-integers, I would not use it that way except for "code golf" (i.e. pointlessly trimming bytes off the source code of your program at the expense of readability and robustness). I would use +x or Number(x) instead.
How this is the NOT of the NOT
The number -43.2, for example is:
-43.210 = 111111111111111111111111110101012
as a signed (two's complement) 32-bit binary number. (JavaScript ignores what is after the decimal point.) Inverting the bits gives:
NOT -4310 = 000000000000000000000000001010102 = 4210
Inverting again gives:
NOT 4210 = 111111111111111111111111110101012 = -4310
This differs from Math.floor(-43.2) in that negative numbers are rounded toward zero, not away from it. (The floor function, which would equal -44, always rounds down to the next lower integer, regardless of whether the number is positive or negative.)
The ~ seems to do -(N+1). So ~2 == -(2 + 1) == -3 If you do it again on -3 it turns it back: ~-3 == -(-3 + 1) == 2 It probably just converts a string to a number in a round-about way.
ReplyDeleteSee this thread: http://www.sitepoint.com/forums/showthread.php?t=663275
Also, more detailed info is available here: http://dreaminginjavascript.wordpress.com/2008/07/04/28/
Given ~N is -(N+1), ~~N is then -(-(N+1) + 1). Which, evidently, leads to a neat trick.
ReplyDeleteThe first ~ operator forces the operand to an integer (possibly after coercing the value to a string or a boolean), then inverts the lowest 31 bits. Officially ECMAScript numbers are all floating-point, but some numbers are implemented as 31-bit integers in the SpiderMonkey engine.
ReplyDeleteYou can use it to turn a 1-element array into an integer. Floating-points are converted according to the C rule, ie. truncation of the fractional part.
The second ~ operator then inverts the bits back, so you know that you will have an integer. This is not the same as coercing a value to boolean in a condition statement, because an empty object {} evaluates to true, whereas ~~{} evaluates to false.
js>~~"yes"
0
js>~~3
3
js>~~"yes"
0
js>~~false
0
js>~~""
0
js>~~true
1
js>~~"3"
3
js>~~{}
0
js>~~{a:2}
0
js>~~[2]
2
js>~~[2,3]
0
js>~~{toString: function() {return 4}}
4
js>~~NaN
0
js>~~[4.5]
4
js>~~5.6
5
js>~~-5.6
-5