js bind 函数 使用闭包保存执行上下文_javascript技巧

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());