Difference between JavaScript deepCopy and shallowCopy

Shallow copy and deep copy are language agnostic. Shallow copies duplicate as little as possible. A shallow copy of a collection is a copy of the collection structure, not the elements. With a shallow copy, two collections now share the individual elements.

Example

let innerObj = {
   a: 'b',
   c: 'd'
}
let obj = {
   x: "test",
   y: innerObj
}
// Create a shallow copy.
let copyObj = Object.assign({}, obj);
// Both copyObj and obj's prop y now refers to the same innerObj. Any changes to this will be reflected.
innerObj.a = "test"
console.log(obj)
console.log(copyObj)

Output

{ x: 'test', y: { a: 'test', c: 'd' } }?
{ x: 'test', y: { a: 'test', c: 'd' } }?

Note that shallow copies do not make clones recursively. It just does it at the top level.

Deep copies duplicate everything. A deep copy of a collection is two collections with all of the elements in the original collection cloned.

Example

let innerObj = {
   a: 'b',
   c: 'd'
}
let obj = {
   x: "test",
   y: innerObj
}
// Create a deep copy.
let copyObj = JSON.parse(JSON.stringify(obj))
// Both copyObj and obj's prop y now refers to the same innerObj. Any changes to this will be reflected.
innerObj.a = "test"
console.log(obj)
console.log(copyObj)

Output

{ x: 'test', y: { a: 'test', c: 'd' } }?
{ x: 'test', y: { a: 'b', c: 'd' } }?