javascript 中的 Math.abs
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/14750182/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Math.abs in javascript
提问by Manoz
I am confused with math.abs(). I did Research through internet but couldn't find any relation with this bouncing ball's animation. I want to know how this works and how ball is bouncing smoothly after using math.abs() function?
我对 math.abs() 感到困惑。我通过互联网进行了研究，但找不到与这个弹跳球的动画有任何关系。我想知道这是如何工作的，以及在使用 math.abs() 函数后球是如何顺利弹跳的？
function bounce() {
if (x + dx > 293 || x + dx < 0) {
dx = -dx;
}
if (y >= 290) {
y = 290;
}
if (y + dy > 290 || y + dy < 0) {
dx *= 0.99;
dy = -dy;
}
//if (Math.abs(dx) < 0.01) {
// dx = 0;
}
dy++;
}
I did commentthe line confusing me. Anyone please let me know that how important this function for this animation.
我确实评论了让我困惑的行。任何人都请让我知道这个功能对这个动画有多重要。
Fiddle
小提琴
回答by Denys Séguret
dxis the displacement on x.
dx是 x 上的位移。
Math.abs(dx)is the absolute speed on x, that is the value without the sign, always positive or null.
Math.abs(dx)是 x 上的绝对速度，即不带符号的值，始终为正或为空。
if (Math.abs(dx) < 0.01) {
could have been written as
可以写成
if (dx>-0.01 && dx < 0.01) {
Basically, this line with the following one stops the ball along xif it's already slow.
基本上，x如果球已经很慢，这条线和以下一条线会停止球。
回答by Rune FS
dx is reduced by 1% every time the last condition is met (if (y + dy > 290 || y + dy < 0) {)
每次满足最后一个条件时，dx 减少 1% ( if (y + dy > 290 || y + dy < 0) {)
this calculation can go on forever but would yield jagged results because the floating point precision errors would become a large factor compared to dx so better stop the ball bouncing when it is already slow which is what the test using Math-abs is for. In english you could read
这种计算可以永远持续下去，但会产生参差不齐的结果，因为与 dx 相比，浮点精度误差将成为一个很大的因素，因此当球已经很慢时最好停止弹跳，这就是使用 Math-abs 的测试的目的。在英语中你可以阅读
if (Math.abs(dx) < 0.01)
as if the speed of the ball in the x direction is less than 0.01 then stop the ball
作为 if the speed of the ball in the x direction is less than 0.01 then stop the ball
