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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]';

- ¶

  Perform a deep comparison to check if two objects are equal.

      export default function isEqual(a, b) {

- ¶

  Keep track of which pairs of values need to be compared. We will be trampolining on this stack instead of using function recursion. (CVE-2026-27601)

        var todo = [{a: a, b: b}];

- ¶

  Initializing stacks of traversed objects for cycle detection.

        var aStack = [], bStack = [];

- ¶

  Keep traversing pairs until there is nothing left to compare.

        while (todo.length) {
          var frame = todo.pop();

- ¶

  As a special case, a single `true` on the todo means that we can unwind the cycle detection stacks.

          if (frame === true) {

- ¶

  Remove the first object from the stack of traversed objects.

            aStack.pop();
            bStack.pop();
            continue;
          }
          a = frame.a;
          b = frame.b;

- ¶

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

          if (a === b) {
            if (a !== 0 || 1 / a === 1 / b) continue;
            return false;
          }

- ¶

  `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) {
            if (b !== b) continue;
            return false;
          }

- ¶

  Exhaust primitive checks

          var type = typeof a;
          if (type !== 'function' && type !== 'object' && typeof b != 'object') return false;

- ¶

  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")`.

            if ('' + a === '' + b) continue;
            return false;
          case '[object Number]':
            todo.push({a: +a, b: +b});
            continue;
          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.

            if (+a === +b) continue;
            return false;
          case '[object Symbol]':
            if (SymbolProto.valueOf.call(a) === SymbolProto.valueOf.call(b)) continue;
            return false;
          case '[object ArrayBuffer]':
          case tagDataView:

- ¶

  Coerce to typed array so we can fall through.

            todo.push({a: toBufferView(a), b: toBufferView(b)});
            continue;
          }

          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) continue;
            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`.

          var length = aStack.length;
          while (length--) {

- ¶

  Linear search. Performance is inversely proportional to the number of unique nested structures.

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

- ¶

  Cycle detected. Break out of the inner loop and continue the outer loop. Step 1:

              if (bStack[length] === b) break;
              return false;
            }
          }

- ¶

  Step 2, use `length` to verify whether we detected a cycle:

          if (length >= 0) continue;

- ¶

  Add the first object to the stack of traversed objects.

          aStack.push(a);
          bStack.push(b);

- ¶

  Remember to remove them again after the recursion below.

          todo.push(true);

- ¶

  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--) {
              todo.push({a: a[length], b: b[length]});
            }
          } 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)) return false;
              todo.push({a: a[key], b: b[key]});
            }
          }
        }

- ¶

  We made it to the end and found no differences.

        return true;
      }