Add how to clone section

[Kikobeats]

This is a popular internet topic.

About:

1. How to clone an array

const newArray = arr.slice(0)
const anotherArray = [...arr]

2. How to clone a plain object

const newObj = Object.assign({}, obj)
const anotherObj = {...obj}

also known as shallow clone:

> third = {hello: 'world'}
> obj = {foo: 'bar', third}
{ foo: 'bar', third: { hello: 'world' } }
> shallow = Object.assign({}, obj)
{ foo: 'bar', third: { hello: 'world' } }
> shallow
{ foo: 'bar', third: { hello: 'world' } }
> third
{ hello: 'world' }
> third.hello = 'test'
'test'
> shallow
{ foo: 'bar', third: { hello: 'test' } }

3. How to clone a deep object and why a simple clone is not enough (talk about passing by value and reference).

const deepClone = obj => {
  let clone = Object.assign({}, obj);
  Object.keys(clone).forEach(
    key => (clone[key] = typeof obj[key] === 'object' ? deepClone(obj[key]) : obj[key])
  );
  return clone;
};

4. Why JSON.parse(JSON.stringify(value)) is not enough.

• Will throw error in circular objects.
• The JSON.stringify conversion has to be lossless. Therefore, methods are not allowed as members of type function are ignored by the JSON stringifier. The undefined value is not allowed either. Object keys with undefined value are omitted, while undefined values in arrays are substituted by null.
• Members of the Date object become ISO-8601 strings, not representing the timezone of the client. The value new Date(2011,0,1) becomes "2010-12-31T23:00:00.000Z" after executing the above deep copy method in case you live in Central Europe.

3. Why not possible do it async style.

parse/stringify is a CPU intensive task, so no I/O is here and not supported from low level Node.js API.
The only "async" way possible is in deepClone wrap each deepClone call into a process.nextTick, simulating deferrer it.

Inspiration:

• https://github.com/juliangruber/proxy-clone
• https://github.com/ahmadnassri/benchmark-node-clone
• https://dassur.ma/things/deep-copy/