JavaScript中對象的比較

Javascript中有'=='和'==='兩種相等比較，後者是全等，會判斷數據類型，前者是相等，在比較時，會發生隱式轉換。

如果將兩個對象做'=='比較，結果會如何呢？

比如有如下兩個對象：

var obj1 = {
    name: "Nicole",
    sex : "female"
}

var obj2 = {
    name: "Nicole",
    sex : "female"
}

//Outputs: false
console.log(obj1 == obj2);

//Outputs: false
console.log(obj1 === obj2);

可以看到，哪怕兩個對象的屬性完全一樣，無論是'=='或者'==='，返回都是false。

原因：對象通過指針指向的記憶體地址來做比較。

繼續上面的例子：

1 var obj3 = obj1;
2 
3 //Outputs: true
4 console.log(obj1 == obj3);
5 
6 //Outputs: true
7 console.log(obj1 === obj3);

如果想根據兩個對象的屬性是否相等，來判斷對象是否相等，可以參考underscore:isEqual(obj1, obj2):

// Internal recursive comparison function for `isEqual`.
    var eq, deepEq;
    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;
        // `null` or `undefined` only equal to itself (strict comparison).
        if (a == null || b == null) return false;
        // `NaN`s are equivalent, but non-reflexive.
        if (a !== a) return b !== b;
        // Exhaust primitive checks
        var type = typeof a;
        if (type !== 'function' && type !== 'object' && typeof b != 'object') return false;
        return deepEq(a, b, aStack, bStack);
    };

    // Internal recursive comparison function for `isEqual`.
    deepEq = function(a, b, aStack, bStack) {
        // 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;
            case '[object Symbol]':
                return SymbolProto.valueOf.call(a) === SymbolProto.valueOf.call(b);
        }

        var areArrays = className === '[object Array]';
        if (!areArrays) {
            if (typeof a != 'object' || typeof b != 'object') return false;

            // Objects with different constructors are not equivalent, but `Object`s or `Array`s
            // from different frames are.
            var aCtor = a.constructor, bCtor = b.constructor;
            if (aCtor !== bCtor && !(_.isFunction(aCtor) && aCtor instanceof aCtor &&
                _.isFunction(bCtor) && bCtor instanceof bCtor)
                && ('constructor' in a && 'constructor' in b)) {
                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`.

        // Initializing stack of traversed objects.
        // It's done here since we only need them for objects and arrays comparison.
        aStack = aStack || [];
        bStack = bStack || [];
        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;
        }

        // Add the first object to the stack of traversed objects.
        aStack.push(a);
        bStack.push(b);

        // Recursively compare objects and arrays.
        if (areArrays) {
            // Compare array lengths to determine if a deep comparison is necessary.
            length = a.length;
            if (length !== b.length) return false;
            // Deep compare the contents, ignoring non-numeric properties.
            while (length--) {
                if (!eq(a[length], b[length], aStack, bStack)) return false;
            }
        } else {
            // Deep compare objects.
            var keys = _.keys(a), key;
            length = keys.length;
            // Ensure that both objects contain the same number of properties before comparing deep equality.
            if (_.keys(b).length !== length) return false;
            while (length--) {
                // Deep compare each member
                key = keys[length];
                if (!(_.has(b, key) && eq(a[key], b[key], aStack, bStack))) return false;
            }
        }
        // Remove the first object from the stack of traversed objects.
        aStack.pop();
        bStack.pop();
        return true;
    };

    // Perform a deep comparison to check if two objects are equal.
    _.isEqual = function(a, b) {
        return eq(a, b);
    };