JS - object && array are by reference only by copy if new array / objects returned such as filter

 // Reference

var a = [{test:1}];

undefined

var b = a ;

undefined

b[0].test = 2

2

b 

[{…}]0: {test: 2}length: 1[[Prototype]]: Array(0)

a 

[{…}]0: {test: 2}length: 1[[Prototype]]: Array(0)

// reference, although filter returns a new array, object still reference

var c = a.filter( item => item.test === 2)

undefined

c

1. [{…}]

c[0].test = 33333

33333

c

1. [{…}]
1. 0: {test: 33333}
2. length: 1
3. [[Prototype]]: Array(0)

a

1. [{…}]
1. 0: {test: 33333}

// Copy

var d = Object.assign({}, a[0])

undefined

d 

{test: 33333}

d.test = 4444444

4444444

d 

{test: 4444444}

a 

[{…}]0: {test: 33333}length: 1[[Prototype]]: Array(0)

// Array.map returns a new array, if non objects.

var a = [1,2,3]

undefined

b = a.map( row=>row)

(3) [1, 2, 3]

b.push(4)

4

b 

(4) [1, 2, 3, 4]

a 

(3) [1, 2, 3]

 // however, if there are objects in the array, they are still by reference 

var a = [{test:1}]

undefined

b = a.map(row=>row)

[{…}]

b[0].test = 22

22

b 

[{…}]0: {test: 22}length: 1[[Prototype]]: Array(0)

a 

[{…}]0: {test: 22}length: 1[[Prototype]]: Array(0)

// To fix use Object.assign in map

var a = [{test:1}]

undefined

b = a.map(row=>Object.assign({},row))

[{…}]

b[0].test = 3333

3333

b 

[{…}]0: {test: 3333}length: 1[[Prototype]]: Array(0)

a

[{…}]0: {test: 1}length: 1[[Prototype]]: Array(0)