Detailed code explanation of how JavaScript compares reference types

项目中有时会需要对引用类型进行比较，如常见的object和array，我们知道，引用类型无法直接使用 == 或=== 取得期待结果，因此需要一个迭代的compare函数转化成原始类型进行比较。在迭代中，我们还要注意的是，对象或者数组中的元素可能是一个任意值——除了原始类型值、object、arrray外，这个值还可能是一个方法、一个DOM对象或者window对象，可能你已经注意到了，有部分引用类型是不能进行迭代的，需要分支判断，代码如下： 

function compare(a,b){ 
var 
pt = /undefined|number|string|boolean/, 
fn = /^(function\s*)(\w*\b)/, 
cr = "constructor", 
cn = "childNodes", 
pn = "parentNode", 
ce = arguments.callee; 
if(pt.test(typeof a) || pt.test(typeof b) || a === null || b === null){ 
return a === b || (isNaN(a) && isNaN(b)); //为了方便，此处假定NaN == NaN 
} 
if(a[cr] !== b[cr]){ 
return false; 
} 
switch(a[cr]){ 
case Date : { 
return a.valueOf() === b.valueOf(); 
}; 
case Function : { 
return a.toString().replace(fn,'$1') === b.toString().replace(fn,'$1'); //硬编码中声明函数的方式会影响到toString的结果，因此用正则进行格式化 
}; 
case Array : { 
if(a.length !== b.length){ 
return false; 
} 
for(var i=0;i<a.length;i++){ 
if(!ce(a[i],b[i])){ 
return false; 
} 
} 
break; 
}; 
default : { 
var alen = 0, blen = 0, d; 
if(a === b){ 
return true; 
} 
if(a[cn] || a[pn] || b[cn] || b[pn]){ 
return a === b; 
} 
for(d in a){ 
alen++ ; 
} 
for(d in b){ 
blen++; 
} 
if(alen !== blen){ 
return false; 
} 
for(d in a){ 
if(!ce(a[d],b[d])){ 
return false; 
} 
} 
break; 
}; 
} 
return true; 
} 
console.log(compare({},{a:1})); //false 
console.log(compare({a:1},{b:2})); //false 
console.log(compare({b:2,a:1},{a:1,b:2})); //true 
console.log(compare({a:function(){return false;},b:2},{a:function(){return false;},b:2})); //true 
console.log(compare([],[])); //true 
console.log(compare([2,1],[1,2])); //false 
console.log(compare(function(){alert(1)},function(){})); //false 
console.log(compare(function aaa(){alert(1)},function(){alert(1)})); //true 
console.log(compare(document.getElementsByTagName("a")[0],document.getElementsByTagName("a")[1])); //false 
console.log(compare(document.getElementsByTagName("a")[0],document.getElementsByTagName("a")[0])); //true

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