在Javascript中，当只声明变量，js默认初始化为undefined
测试如下：
var r;
alert(Object.prototype.toString.call(null)); //< [object Null]
alert(Object.prototype.toString.call(undefined)); //< [object Undefined]
alert(Object.prototype.toString.call(r)); //< [object Undefined]
alert(typeof null); //< object
alert(typeof undefined); //< undefined
alert(typeof r); //< undefined
alert(null == undefined); //< true
alert(null == r); //< 默认对r进行类型转换 true
alert(undefined == r); //< true
alert(null === undefined); //< false
alert(null === r); //< false
alert(undefined === r); //< true
所以参考prototype.js中的代码：
function min(iterator, context) {
iterator = iterator || prototype.k;
var result;
this.each(function(value, index) {
value = iterator.call(context, value, index);
if (result == null || value < result)
result = value;
});
return result;
}
为了避免默认的类型转换：
function min(iterator, context) {
iterator = iterator || prototype.k;
var result;
this.each(function(value, index) {
value = iterator.call(context, value, index);
if (result === undefined || value < result)
result = value;
});
return result;
}