JavaScript中如何判断两变量是否“相等”？

0 === 0
//true
+0 === -0
//true

null == undefined
//true
null === undefined
//false

NaN == NaN
//false
NaN === NaN
//false   

var a = [];
//undefined
var b = [];
//undefined
a=== b
//false
a==b
//false

var a  = {};
//undefined
var b = {};
//undefined
a == b
//false
a === b
//false

2 == new Number(2);
//true
2 === new Number(2);
//false

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).
	//除了0 === -0这个特例之外，其余所有a === b的例子都代表它们相等。
	//应当判断0 !== -0，但是JavaScript中0 === -0。
	//下面这行代码就是为了解决这个问题。
	//当a !== 0或者1/a === 1/b时返回true，一旦a === 0并且1/a !== 1/b就返回false。
	//而a === 0且1/a !== 1/b就代表a，b有一个为0，有一个为-0。
	if (a === b) return a !== 0 || 1 / a === 1 / b;
	//一旦a、b不严格相等，就进入后续检测。
	//a == b成立但是a === b不成立的例子中需要排除null和undefined，其余例子需要后续判断。
	// `null` or `undefined` only equal to itself (strict comparison).
	//一旦a或者b中有一个为null就代表另一个为undefined，这种情况可以直接排除。
	if (a == null || b == null) return false;
	// `NaN`s are equivalent, but non-reflexive.
	//自身不等于自身的情况，一旦a，b都为NaN，则可以返回true。
	if (a !== a) return b !== b;
	// Exhaust primitive checks
	//如果a，b都不为JavaScript对象，那么经过以上监测之后还不严格相等的话就可以直接断定a不等于b。
	var type = typeof a;
	if (type !== ‘function‘ && type !== ‘object‘ && typeof b != ‘object‘) return false;
	//如果a，b是JavaScript对象，还需要做后续深入的判断。
	return deepEq(a, b, aStack, bStack);
};

var deepEq = function (a, b, aStack, bStack) {
	// Unwrap any wrapped objects.
	//如果a，b是_的一个实例的话，需要先把他们解包出来再进行比较。
	if (a instanceof _) a = a._wrapped;
	if (b instanceof _) b = b._wrapped;
	// Compare `[[Class]]` names.
	//先根据a，b的Class字符串进行比较，如果两个对象的Class字符串都不一样，
	//那么直接可以认为两者不相等。
	var className = toString.call(a);
	if (className !== toString.call(b)) return false;
	//如果两者的Class字符串相等，再进一步进行比较。
	//优先检测内置对象之间的比较，非内置对象再往后检测。
	switch (className) {
		// Strings, numbers, regular expressions, dates, and booleans are compared by value.
		//如果a，b为正则表达式，那么转化为字符串判断是否相等即可。
		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")`.
			//如果a， b是字符串对象，那么转化为字符串进行比较。因为一下两个变量：
			//var x = new String(‘12‘);
			//var y = new String(‘12‘);
			//x === y是false，x === y也是false，但是我们应该认为x与y是相等的。
			//所以我们需要将其转化为字符串进行比较。
			return ‘‘ + a === ‘‘ + b;
		case ‘[object Number]‘:
			//数字对象转化为数字进行比较，并且要考虑new Number(NaN) === new Number(NaN)应该要成立的情况。
			// `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.
			//排除0 === -0 的情况。
			return +a === 0 ? 1 / +a === 1 / b : +a === +b;
		case ‘[object Date]‘:
		//Date类型以及Boolean类型都可以转换为number类型进行比较。
		//在变量前加一个加号“+”，可以强制转换为数值型。
		//在Date型变量前加一个加号“+”可以将Date转化为毫秒形式；Boolean类型同上（转换为0或者1）。
		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]‘;
	//如果不是数组对象。
	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.
		//比较两个非数组对象的构造函数。
		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.
	//初次调用eq函数时，aStack以及bStack均未被传递，在循环递归的时候，会被传递进来。
	//aStack和bStack存在的意义在于循环引用对象之间的比较。
	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.
	//初次调用eq函数时，就把两个参数放入到参数堆栈中去，保存起来方便递归调用时使用。
	aStack.push(a);
	bStack.push(b);

	// Recursively compare objects and arrays.
	//如果是数组对象。
	if (areArrays) {
		// Compare array lengths to determine if a deep comparison is necessary.
		length = a.length;
		//长度不等，直接返回false认定为数组不相等。
		if (length !== b.length) return false;
		// Deep compare the contents, ignoring non-numeric properties.
		while (length--) {
			//递归调用。
			if (!eq(a[length], b[length], aStack, bStack)) return false;
		}
	} else {
		// Deep compare objects.
		//对比纯对象。
		var keys = _.keys(a), key;
		length = keys.length;
		// Ensure that both objects contain the same number of properties before comparing deep equality.
		//对比属性数量，如果数量不等，直接返回false。
		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.
	//循环递归结束，把a，b堆栈中的元素推出。
	aStack.pop();
	bStack.pop();
	return true;
};

var a = {
    name: ‘test‘
};
a[‘test1‘] = a;
var b = {
    name: ‘test‘
};
b[‘test1‘] = b;
_.isEqual(a, b);
//true

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;
}

var size = a.size;
// Ensure that both objects are of the same size before comparing deep equality.
if (b.size !== size) return false;
while (size--) {
    // Deep compare the keys of each member, using SameValueZero (isEq) for the keys
    if (!(isEq(a.keys().next().value, b.keys().next().value, aStack, bStack))) return false;
    // If the objects are maps deep compare the values. Value equality does not use SameValueZero.
    if (className === ‘[object Map]‘) {
        if (!(eq(a.values().next().value, b.values().next().value, aStack, bStack))) return false;
    }
}