Not sure if this is intended, it doesn't match what I use _.cloneDeep for but perhaps it's desired for other cases. It came up unexpectedly in our codebase so I'd be curious—if it's intended behavior—if it could be documented clearly.

It seems that _.cloneDeep maintains, in the cloned object, two keys referencing the same object in the original object.

Example case:

'use strict'

let _ = require('lodash')

let x = { lodash: 'isgreat' }

let y = { 
    one: x,
    two: x,
    three: { 
        javascript: 'isfun' 
    }
}

console.log('One and two are equal in original object?', y.one === y.two)

let yy = _.cloneDeep(y)

console.log('One and two are equal in deep cloned object?', yy.one === yy.two)
console.log('One is different object before and after clone?', y.one === yy.one)

This prints:

One and two are equal in original object? true
One and two are equal in deep cloned object? true
One is different object before and after clone? false

I would have expected it to print:

One and two are equal in original object? true
One and two are equal in deep cloned object? false
One is different object before and after clone? false