How do I compare two objects in JavaScript?

Equality comparison

Even though two different objects can have the same properties with equal values, they are not considered equal when compared using either the loose or strict equality operators (== or ===). This is because arrays and objects in JavaScript are compared by reference. This is unlike primitive values which are compared by value.

const a = { name: 'John', age: 26 };
const b = { name: 'John', age: 26 };

a === b; // false

JSON.stringify

JSON.stringify() often comes up as the solution to this problem. While it can be useful in some situations, comparing the serialized strings can have its own pitfalls. The most common of these has to do with similar, but not equal, values that result in the same serialized string.

const equals = (a, b) => JSON.stringify(a) === JSON.stringify(b);

const a = { name: 'John', age: 26 };
const b = { name: 'John', age: 26 };

equals(a, b); // true

const c = { name: 'John' };
const d = { name: 'John', age: undefined };

equals(c, d); // true, should be false

Deep equality comparison

As it turns out, comparing two objects is not trivial. This is the reason why shallow or deep equality comparison helper functions are so common. These usually use recursion to deeply compare two objects, accounting for most scenarios such as empty values, special types and nesting.

In order to implement such a function, we need to account for various scenarios. Using recursion, we will then compare the values of the two objects at each level of nesting.

Firstly, we check if the two values are identical. This will account for primitive values and references to the same object. Then, we take care of the case where both values are Date objects. We compare their time values using Date.prototype.getTime().

After that, we can check if both values are non-object values with an equivalent value. This is done using strict comparison. If only one value is null or undefined, or if their prototypes differ, we return false.

If none of the above conditions are met, we use Object.keys() to check if both values have the same number of keys. We then use Array.prototype.every() to check if every key in a exists in b and if they are equivalent by calling equals() recursively.

const equals = (a, b) => {
  if (a === b) return true;

  if (a instanceof Date && b instanceof Date)
    return a.getTime() === b.getTime();

  if (!a || !b || (typeof a !== 'object' && typeof b !== 'object'))
    return a === b;

  if (a.prototype !== b.prototype) return false;

  const keys = Object.keys(a);
  if (keys.length !== Object.keys(b).length) return false;

  return keys.every(k => equals(a[k], b[k]));
};

const a = { name: 'John', age: 26 };
const b = { name: 'John', age: 26 };

equals(a, b); // true

const c = { name: 'John' };
const d = { name: 'John', age: undefined };

equals(c, d); // false

const e = [1, 2, 3];
const f = { 0: 1, 1: 2, 2: 3 };

equals(e, f); // true