69. implement deep equal `_.isEqual()`

medium  - accepted / - tried

_.isEqual is useful when you want to compare complex data types by value not the reference.

Can you implement your own version of deep equal isEqual? The lodash version covers a lot of data types. In this problem, you are asked to support :

  1. primitives
  2. plain objects (object literals)
  3. array

Objects are compared by their own, not inherited, enumerable properties

const a = {a: 'bfe'}
const b = {a: 'bfe'}

isEqual(a, b) // true
a === b // false

const c = [1, a, '4']
const d = [1, b, '4']

isEqual(c, d) // true
c === d // false

Lodash implementation has some strange behaviors. (github issue, like following code

const a = {}
a.self = a
const b = {self: a}
const c = {}
c.self = c
const d = {self: {self: a}}
const e = {self: {self: b}}

lodash.isEqual gives us following result. Notice there is a case that resulting in false.

// result from lodash implementation
_.isEqual(a, b) // true
_.isEqual(a, c) // true
_.isEqual(a, d) // true
_.isEqual(a, e) // true
_.isEqual(b, c) // true
_.isEqual(b, d) // true
_.isEqual(b, e) // false
_.isEqual(c, d) // true
_.isEqual(c, e) // true
_.isEqual(d, e) // true

Setting aside the performance concerns mentioned by lodash, your implement should not have above problem, which means above all returns true and call stack should not exceed the maximum.

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