JavaScript Interview : Define DeepClone Function

In JavaScript, we have primitive types(undefined, string, number, boolean, Symbol) and reference types(object,array,function). Primitive types store in stack; memory is fixed; store the actual value while reference types store in heap; memory is not fixed; store the address of value.

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Cope a primitive value is called deep clone and copy a primitive value is called shallow clone which means when we change the clone value we also change the original value:

// deep copy: primitive type
let a = 10
let  b = a
b = 20
console.log(a) // 10

//shallow copy: reference type
let obj1 = {
name:'Alice',
age:12
}
let obj2 = obj1
obj2.age = 20
console.log(obj1.age) // 20

Here is an interesting interview question:

let obj1 = {
name:'Alice',
age:12
}
let obj2 = obj1
let age = obj1.age
obj2.age = 15
age = 20
console.log(obj2.age) // 15

It combines the reference types and primitive types. When we define the age, it’s a primitive type and independent from the object.

How to deep clone an object ? Here is a solution:

function DeepClone(obj={}){
  if(typeof obj !== 'object' || obj === null){
// use tripple equles here is better
// since typeof null equals to 'object'
    return obj
  }
  let result
  if(obj instanceof Array){
    result = []
  }else{
    result = {}
  }

  for(let key in obj){
    if(obj.hasOwnProperty(key)){
      result[key] = DeepClone(obj[key])
    }

  }
  return result

}
const obj1 = {'age' : 12}
const obj2 = DeepClone(obj1)
obj2.age = 15
console.log(obj1.age) // 12