js bind 函数 使用闭包保存执行上下文

window.name = "the window object" 
function scopeTest() { 
return this.name; 
} 
// calling the function in global scope: 
scopeTest() 
// -> "the window object" 
var foo = { 
name: "the foo object!", 
otherScopeTest: function() { return this.name } 
}; 
foo.otherScopeTest();// -> "the foo object!" 
var foo_otherScopeTest = foo.otherScopeTest; 
foo_otherScopeTest(); 
// –> "the window object"

如果你希望将一个对象的函数赋值给另外一个变量后，这个函数的执行上下文仍然为这个对象，那么就需要用到bind方法。
bind的实现如下：

// The .bind method from Prototype.js 
Function.prototype.bind = function(){ 
var fn = this, args = Array.prototype.slice.call(arguments), object = args.shift(); 
return function(){ 
return fn.apply(object, 
args.concat(Array.prototype.slice.call(arguments))); 
}; 
};

使用示例：

var obj = { 
name: 'A nice demo', 
fx: function() { 
alert(this.name); 
} 
}; 
window.name = 'I am such a beautiful window!'; 
function runFx(f) { 
f(); 
} 
var fx2 = obj.fx.bind(obj); 
runFx(obj.fx); 
runFx(fx2);

参考： http://www.prototypejs.org/api/function/bind
PS：
才发现prototypejs的API文档解释的这么详细，一定要花点时间多看看了。
我的简单的实现：

Function.prototype.bind = function(obj) { 
var _this = this; 
return function() { 
return _this.apply(obj, 
Array.prototype.slice.call(arguments)); 
} 
} 
var name = 'window', 
foo = { 
name:'foo object', 
show:function() { 
return this.name; 
} 
}; 
console.assert(foo.show()=='foo object', 
'expected foo object,actual is '+foo.show()); 
var foo_show = foo.show; 
console.assert(foo_show()=='window', 
'expected is window,actual is '+foo_show()); 
var foo_show_bind = foo.show.bind(foo); 
console.assert(foo_show_bind()=='foo object', 
'expected is foo object,actual is '+foo_show_bind());