javascript 判断对像是否相等

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：

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：

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：

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，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：

// Outputs: true
console.log(_.isEqual(obj1, obj2));

最后附上Underscore中isEqual的部分源码：

// 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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