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- # isEqual.js

- ¶

      import _ from './underscore.js';
      import { toString, SymbolProto } from './_setup.js';
      import getByteLength from './_getByteLength.js';
      import isTypedArray from './isTypedArray.js';
      import isFunction from './isFunction.js';
      import { hasDataViewBug }  from './_stringTagBug.js';
      import isDataView from './isDataView.js';
      import keys from './keys.js';
      import has from './_has.js';
      import toBufferView from './_toBufferView.js';

- ¶

  We use this string twice, so give it a name for minification.

      var tagDataView = '[object DataView]';

- ¶

  Internal recursive comparison function for `_.isEqual`.

      function eq(a, b, aStack, bStack) {

- ¶

  Identical objects are equal. `0 === -0`, but they aren’t identical. See the Harmony `egal` proposal.

        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`.

      function deepEq(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;

- ¶

  Work around a bug in IE 10 - Edge 13.

        if (hasDataViewBug && className == '[object Object]' && isDataView(a)) {
          if (!isDataView(b)) return false;
          className = tagDataView;
        }
        switch (className) {

- ¶

  These types 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);
          case '[object ArrayBuffer]':
          case tagDataView:

- ¶

  Coerce to typed array so we can fall through.

            return deepEq(toBufferView(a), toBufferView(b), aStack, bStack);
        }

        var areArrays = className === '[object Array]';
        if (!areArrays && isTypedArray(a)) {
            var byteLength = getByteLength(a);
            if (byteLength !== getByteLength(b)) return false;
            if (a.buffer === b.buffer && a.byteOffset === b.byteOffset) return true;
            areArrays = true;
        }
        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.

      export default function isEqual(a, b) {
        return eq(a, b);
      }