在Javascript中相等运算包括"==","==="全等，两者不同之处，不必多数，本篇文章我们将来讲述如何判断两个对象是否相等？ 你可能会认为，如果两个对象有相同的属性，以及它们的属性有相同的值，那么这两个对象就相等。那么下面我们通过一个实例来论证下：
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var obj1 = { name: "Benjamin", sex : "male"}var obj2 = { name: "Benjamin", sex : "male"}//Outputs: falseconsole.log(obj1 == obj2);//Outputs: falseconsole.log(obj1 === obj2); |
通过上面的例子可以看到，无论使用"=="还是"==="，都返回false。主要原因是基本类型string,number通过值来比较，而对象（Date,Array）及普通对象通过指针指向的内存中的地址来做比较。看下面一个例子：
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var obj1 = { name: "Benjamin", sex : "male"};var obj2 = { name: "Benjamin", sex : "male"};var obj3 = obj1;//Outputs: trueconsole.log(obj1 == obj3);//Outputs: trueconsole.log(obj1 === obj3);//Outputs: falseconsole.log(obj2 == obj3);//Outputs: falseconsole.log(obj2 === obj3); |
上例返回true，是因为obj1和ob3的指针指向了内存中的同一个地址。和面向对象的语言（Java/C++）中值传递和引用传递的概念相似。 因为，如果你想判断两个对象是否相等，你必须清晰，你是想判断两个对象的属性是否相同，还是属性对应的值是否相同，还是怎样？如果你判断两个对象的值是否相等，可以像下面这样：
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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: trueconsole.log(isObjectValueEqual(obj1, obj2)); |
正如你所看到的，检查对象的“值相等”我们基本上是要遍历的对象的每个属性，看看它们是否相等。虽然这个简单的实现适用于我们的例子中，有很多情况下，它是不能处理。例如： 1) 如果该属性值之一本身就是一个对象吗？ 2) 如果属性值中的一个是NaN（在JavaScript中，是不是等于自己唯一的价值？） 3) 如果一个属性的值为undefined，而另一个对象没有这个属性（因而计算结果为不确定？） 检查对象的“值相等”的一个强大的方法，最好是依靠完善的测试库，涵盖了各种边界情况。Underscore和Lo-Dash有一个名为_.isEqual()方法，用来比较好的处理深度对象的比较。您可以使用它们像这样：
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// Outputs: trueconsole.log(_.isEqual(obj1, obj2)); |
最后附上Underscore中isEqual的部分源码：
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// 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, [], []);}; |
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