在Javascript中相等運算包括"==","==="全等，兩者不同之處，不必多數，本篇文章我們將來講述如何判斷兩個對象是否相等？ 你可能會認為，如果兩個對象有相同的屬性，以及它們的屬性有相同的值，那么這兩個對象就相等。那么下面我們通過一個實例來論證下：
var obj1 = {
name: "Benjamin",
sex : "male"
}
var obj2 = {
name: "Benjamin",
sex : "male"
}
//Outputs: false
console.log(obj1 == obj2);
//Outputs: false
console.log(obj1 === obj2);
通過上面的例子可以看到，無論使用"=="還是"==="，都返回false。主要原因是基本類型string,number通過值來比較，而對象（Date,Array）及普通對象通過指針指向的內存中的地址來做比較。看下面一個例子：
var obj1 = {
name: "Benjamin",
sex : "male"
};
var obj2 = {
name: "Benjamin",
sex : "male"
};
var obj3 = obj1;
//Outputs: true
console.log(obj1 == obj3);
//Outputs: true
console.log(obj1 === obj3);
//Outputs: false
console.log(obj2 == obj3);
//Outputs: false
console.log(obj2 === obj3);
上例返回true，是因為obj1和ob3的指針指向了內存中的同一個地址。和面向對象的語言（Java/C++）中值傳遞和引用傳遞的概念相似。 因為，如果你想判斷兩個對象是否相等，你必須清晰，你是想判斷兩個對象的屬性是否相同，還是屬性對應的值是否相同，還是怎樣？如果你判斷兩個對象的值是否相等，可以像下面這樣：
function isObjectValueEqual(a, b) {
// Of course, we can do it use for in
// Create arrays of property names
var aProps = Object.getOwnPropertyNames(a);
var bProps = Object.getOwnPropertyNames(b);
// If number of properties is different,
// objects are not equivalent
if (aProps.length != bProps.length) {
return false;
}
for (var i = 0; i < aProps.length; i++) {
var propName = aProps[i];
// If values of same property are not equal,
// objects are not equivalent
if (a[propName] !== b[propName]) {
return false;
}
}
// If we made it this far, objects
// are considered equivalent
return true;
}
var obj1 = {
name: "Benjamin",
sex : "male"
};
var obj2 = {
name: "Benjamin",
sex : "male"
};
//Outputs: true
console.log(isObjectValueEqual(obj1, obj2));
正如你所看到的，檢查對象的“值相等”我們基本上是要遍歷的對象的每個屬性，看看它們是否相等。雖然這個簡單的實現適用於我們的例子中，有很多情況下，它是不能處理。例如： 1) 如果該屬性值之一本身就是一個對象嗎？ 2) 如果屬性值中的一個是NaN（在JavaScript中，是不是等於自己唯一的價值？） 3) 如果一個屬性的值為undefined，而另一個對象沒有這個屬性（因而計算結果為不確定？） 檢查對象的“值相等”的一個強大的方法，最好是依靠完善的測試庫，涵蓋了各種邊界情況。Underscore和Lo-Dash有一個名為_.isEqual()方法，用來比較好的處理深度對象的比較。您可以使用它們像這樣：
// Outputs: true console.log(_.isEqual(obj1, obj2));
最后附上Underscore中isEqual的部分源碼：
// Internal recursive comparison function for `isEqual`.
var eq = function(a, b, aStack, bStack) {
// Identical objects are equal. `0 === -0`, but they aren't identical.
// See the [Harmony `egal` proposal](http://wiki.ecmascript.org/doku.php?id=harmony:egal).
if (a === b) return a !== 0 || 1 / a === 1 / b;
// A strict comparison is necessary because `null == undefined`.
if (a == null || b == null) return a === b;
// Unwrap any wrapped objects.
if (a instanceof _) a = a._wrapped;
if (b instanceof _) b = b._wrapped;
// Compare `[[Class]]` names.
var className = toString.call(a);
if (className !== toString.call(b)) return false;
switch (className) {
// Strings, numbers, regular expressions, dates, and booleans are compared by value.
case '[object RegExp]':
// RegExps are coerced to strings for comparison (Note: '' + /a/i === '/a/i')
case '[object String]':
// Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
// equivalent to `new String("5")`.
return '' + a === '' + b;
case '[object Number]':
// `NaN`s are equivalent, but non-reflexive.
// Object(NaN) is equivalent to NaN
if (+a !== +a) return +b !== +b;
// An `egal` comparison is performed for other numeric values.
return +a === 0 ? 1 / +a === 1 / b : +a === +b;
case '[object Date]':
case '[object Boolean]':
// Coerce dates and booleans to numeric primitive values. Dates are compared by their
// millisecond representations. Note that invalid dates with millisecond representations
// of `NaN` are not equivalent.
return +a === +b;
}
if (typeof a != 'object' || typeof b != 'object') return false;
// Assume equality for cyclic structures. The algorithm for detecting cyclic
// structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.
var length = aStack.length;
while (length--) {
// Linear search. Performance is inversely proportional to the number of
// unique nested structures.
if (aStack[length] === a) return bStack[length] === b;
}
// Objects with different constructors are not equivalent, but `Object`s
// from different frames are.
var aCtor = a.constructor, bCtor = b.constructor;
if (
aCtor !== bCtor &&
// Handle Object.create(x) cases
'constructor' in a && 'constructor' in b &&
!(_.isFunction(aCtor) && aCtor instanceof aCtor &&
_.isFunction(bCtor) && bCtor instanceof bCtor)
) {
return false;
}
// Add the first object to the stack of traversed objects.
aStack.push(a);
bStack.push(b);
var size, result;
// Recursively compare objects and arrays.
if (className === '[object Array]') {
// Compare array lengths to determine if a deep comparison is necessary.
size = a.length;
result = size === b.length;
if (result) {
// Deep compare the contents, ignoring non-numeric properties.
while (size--) {
if (!(result = eq(a[size], b[size], aStack, bStack))) break;
}
}
} else {
// Deep compare objects.
var keys = _.keys(a), key;
size = keys.length;
// Ensure that both objects contain the same number of properties before comparing deep equality.
result = _.keys(b).length === size;
if (result) {
while (size--) {
// Deep compare each member
key = keys[size];
if (!(result = _.has(b, key) && eq(a[key], b[key], aStack, bStack))) break;
}
}
}
// Remove the first object from the stack of traversed objects.
aStack.pop();
bStack.pop();
return result;
};
// Perform a deep comparison to check if two objects are equal.
_.isEqual = function(a, b) {
return eq(a, b, [], []);
};
感謝您的閱讀，希望此篇文章對您有所幫助，文中不足之處歡迎批評斧正。
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