G法师的博客

[]==false之原理

城晓 — Sat, 10 Jul 2010 00:49:23 +0000

问题来源

var a = [];

if(a) alert("true");//true

if(a == true) alert("true");//false

原理

js引擎会先计算括号内的表达式然后将结果转换为布尔值，所以

if(a) => if(!!a) => true

if(a == true) => if(Number(a) == true) => if(0 == true) => true

注: 在比较的两个值中，如果有一个是布尔类型，那么另外一个值将会先被转换为数字类型再与其比较。

参考：双等号的比较原理

所以，下面这个就会有看起来奇怪的结果

[] == ![] //true

超长叠代

城晓 — Thu, 17 Jun 2010 10:29:09 +0000

for(var i=0; i<1000000; i++) {
    //do some thing.
}

话说以上代码很可能会被浏览器判作执行超时。

试验了一下可以用浏览器的延时来做：

(function() {
    //do some thing.
    setTimeout(arguments.callee, 0);
})();

这样一来CPU倒是基本没有，但是这样的代码运行过慢，所以综合一下：

-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
http://www.w3.org/1999/xhtml">
 
 Content-Type" content="text/html; charset=utf-8" />
   <span style="color: #0000ff">new</span> <span style="color: #0000ff">document</span> 
  
 

 
  text" id="log" />

注意运行时也会使浏览器无反应，不过在每一个任务队列结束时仍然有机会刷新面。

MYSQL导入与导出命令

城晓 — Mon, 07 Jun 2010 14:48:07 +0000

可以备份大量数据

1，数据库备份命令
mysqldump -h localhost -u root -p --default-character-set=utf8 dbname >backup.sql

2，导入数据命令：
mysql -h localhost -u root -p --default-character-set=utf8
use dbname
source backup.sql

双等号的比较原理

城晓 — Sun, 09 May 2010 05:42:27 +0000

直接摘自ECMA262

章节号：11.9.3 The Abstract Equality Comparison Algorithm

比较 x == y, 当 x 和 y 都是值的时候, 产生true或false. 这样的一个比较执行如下：

If Type(x) is the same as Type(y), then
1. If Type(x) is undefined, return true.
2. If Type(x) is Null, return true.
3. If Type(x) is Number, then
  1. If x is NaN, return false.
  2. If y is NaN, return false.
  3. If x is the same Number value as y, return true.
  4. If x is +0 and y is −0, return true.
  5. If x is −0 and y is +0, return true.
  6. Return false.
4. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
5. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
6. Return true if x and y refer to the same object. Otherwise, return false.
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
If Type(x) is String and Type(y) is Number,
return the result of the comparison ToNumber(x) == y.
If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
If Type(x) is either String or Number and Type(y) is Object,
return the result of the comparison x == ToPrimitive(y).
If Type(x) is Object and Type(y) is either String or Number,
return the result of the comparison ToPrimitive(x) == y.
Return false.

NOTE 1 鉴于以上相等的定义：
• 可以这样强制字符串比较： "" + a == "" + b.
• 可以这样强制数字比较： +a == +b.
• 可以这样强制布尔值比较：!a == !b.
NOTE 2 双等比较保持以下的不变式：
• A != B 等价于 !(A == B).
• A == B 等价于 B == A, 除了A和B求值的顺序不同
NOTE 3 The equality operator is not always transitive. For example, there might be two distinct String objects, each
representing the same String value; each String object would be considered equal to the String value by the == operator,
but the two String objects would not be equal to each other.
NOTE 4 Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to
use the more complex, semantically oriented definitions of character or string equality and collating order defined in the
Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form.

最近这部分待译

自定义事件之双击按键

城晓 — Thu, 15 Apr 2010 14:56:04 +0000

遇到这样的需求，双击按键的事件
还是直接上代码吧：

//首先准备好常规的事件绑定函数
function addEvent(element, name, callback) {
	//标准浏览器
	if(element.addEventListener) element.addEventListener(name, callback, false);
	//IE
	else element.attachEvent("on" + name, callback);
}
//双击按键绑定函数
function addDblPressEvent(element, callback) {
	var presstime = 2;//次数为2
	addEvent(element, "keyup", press);
	//每次单击触发
	function press(e) {
		presstime--;
		setTimeout(reset, 300);
		if(presstime === 0) fire(e);
	}
	//恢复计数器
	function reset() {
		presstime++;
	}
	//触发双击
	function fire(e) {
		callback.call(element, e);
	}
}

//试一下
window.onload = function() {
	addDblPressEvent(document, function(e) {
		console.log("double clicked");
	});
}

system

城晓 — Mon, 12 Apr 2010 13:49:47 +0000

我忘了是否从哪里看过，或者是否是经典，仅仅是突然想到的，就是这样的

系统

function.delay

城晓 — Wed, 31 Mar 2010 13:27:00 +0000

扩展一个函数的延时调用

Function.prototype.delay =  function(n, thisObj, args) {
    var me = this;
    args = args instanceof Array ? args : [];
    var f = function() {
        me.apply(thisObj || window, args);
    };
    window.setTimeout(f, n);
}
//示例
function f() {
    alert("hi");
}
alert("两秒之后弹出");
f.delay(2000);

注意第三个是参数数组

第一次用Live Writer写日志，呵呵

各浏览器的鼠标键盘事件整理

城晓 — Sun, 28 Mar 2010 14:19:12 +0000

这是Window XP(sp3)下的测试结果，如下表：

Key Event
(windows XP)
浏览器	一次触发		连续触发
	控制键	非控制键	控制键	非控制键
IE8/8.0.6001	keydown keyup	keydown keypress keyup	keydown	keydown keypress
FireFox/3.6.2	keydown keyup	keydown keypress keyup	keydown	keydown keypress
Chrome/4.1.249.1042	keydown keyup	keydown keypress keyup	keydown	keydown keypress
Safari/4.0.3	keydown keyup	keydown keypress keyup	keydown	keydown keypress
Opera/10.51	keydown keyup	keydown keypress keyup	-	keypress

Mouse Event
(windows XP)
浏览器	单击	双击
IE8/8.0.6001	mousedown mouseup click	mousedown mouseup click mouseup dblclick
FireFox/3.6.2	mousedown mouseup click	mousedown mouseup click mousedown mouseup click dblclick
Chrome/4.1.249.1042	mousedown mouseup click	mousedown mouseup click mousedown mouseup click dblclick
Safari/4.0.3	mousedown mouseup click	mousedown mouseup click mousedown mouseup click dblclick
Opera/10.51	mousedown mouseup click	mousedown mouseup click mousedown mouseup click dblclick

浏览器中的key event--ubuntu

城晓 — Sun, 21 Mar 2010 10:11:47 +0000

测试了一下ubuntu中的浏览器对于键盘事件的触发情况，结果如下：

Key Event
(Ubuntu9.10)
浏览器	一次触发		连续触发
	控制键	非控制键	控制键	非控制键
FireFox/3.6	keydown keyup	keydown keypress keyup	-	keydown keypress keyup
Chromium/5.0.318.0	keydown keyup	keydown keyup	-	keydown keyup
Opera/10.10	keydown keyup	keydown keypress keyup	-	keypress

学习canvas

城晓 — Fri, 19 Mar 2010 06:10:57 +0000

创建了这个对象以后我们就可以用js来在上面画东西了

首先取得这个对象的引用

var canvas = document.getElementById("canvas");
然后取得绘制对象
var ctx = canvas.getContext("2d");

现在用ctx画一条直线

先把“画笔”放到起始点（0, 0）

ctx.moveTo(0, 0);

然后画线到（50, 50）

ctx.lineTo(50, 50);

把线画出来
ctx.stroke();

就这么简单，更多的测试看 canvas测试页
 Canvas tutorial