js中两个对象的比较

时间：2021-11-24 14:59:31

代码取自于underscore.js 1.8.3的isEqual函数。

做了一些小小的修改，主要是Function的比较修改。

自己也加了一些代码解读。

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <title>js中两个对象的比较</title>

    <script>

/*

需求难点描述：

数组和对象，都能包含自身，还能包含其它类型。

所以数组之间的比较，要递归。

所以这块代码的设计是：

不能包含自身的，先比较。

然后是数组，对象比较。

特别要注意的是，对象的循环引用。

递归时，要记录递归的路径。

*/

function isEqual(a, b) {

  var toString = Object.prototype.toString,

    object_keys = Object.keys,

    has = function(obj, key) {

      return obj != null && hasOwnProperty.call(obj, key);

    };

    var isFunction = function(fn){

        return toString.call(fn) == '[object Function]' ? true : 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).

    // 验证0===-0

    if (a === b) return a !== 0 || 1 / a === 1 / b;

    // A strict comparison is necessary because `null == undefined`.

    // null

    // 验证null == undefined

    if (a == null || b == null) return a === b;

    // 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]':

        // function虽然是引用，但两个内容一样的funciton应该相等。

      case '[object Function]':

        // 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

        // 验证 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;

    }

    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;

        // 判断顺序

        // 构造器一致

        // 构造器为函数

        // 拥有构造器属性

        // Function instanceof Function == true

        // Object instanceof Object == true

        // Array instanceof Array == false

      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--) {

      // 递归才会走到这步

       // Linear search. Performance is inversely proportional to the number of

      // unique nested structures.

      // 检测循环引用，参考用例如下

      /*

      var a = {

        "str":"string",

      };

      a["test"]=a;

      var b = {

        "str":"string",

      };

      b["test"]=b;

      console.log (isEqual(a,b));

      */

      if (aStack[length] === a){

          // 判断b的引用是否也循环，跳出循环引用这个坑

        return bStack[length] === b;

      }

    }

    // Add the first object to the stack of traversed objects.

    // 递归压栈

    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;

      // 数组长度比较

      if (length !== b.length) return false;

      // Deep compare the contents, ignoring non-numeric properties.

      // 递归比较

      while (length--) {

        if (!eq(a[length], b[length], aStack, bStack)) return false;

      }

    } else {

      // Deep compare objects.

      // 把对象的属性们转换成一个数组

      var keys = object_keys(a),

        key;

      length = keys.length;

      // Ensure that both objects contain the same number of properties before comparing deep equality.

      if (object_keys(b).length !== length) return false;

      while (length--) {

        // Deep compare each member

        key = keys[length];

        // b也有a一样的key，则递归

        if (!(has(b, key) && eq(a[key], b[key], aStack, bStack))) return false;

      }

    }

    // Remove the first object from the stack of traversed objects.

    // 递归出栈

    aStack.pop();

    bStack.pop();

    return true;

  };

  return eq(a,b);

}

var a = {

  "str":"ying",

};

a["test"]=a;

var b = {

  "str":"ying",

};

b["test"]=a;

console.log (isEqual(a,b));

    </script>

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