Copy object properties maintaining mutability

Question

I need to copy some properties of an object to another object and maintain its mutability.

Here is the source object:

var originalObj = {
  "key1": "value1",
  "key2": "value2",
  "nestedObj1" : {
    "nestedKey1" : "nestedValue1",
    "nestedKey2" : "nestedValue2"
  },
  "nestedArr1" : [
    {
      "nestedKey3" : "nestedValue3",
      "nestedKey4" : "nestedValue4"
    },
    {
      "nestedKey5" : "nestedValue5",
      "nestedKey6" : "nestedValue6"
    }
  ],
  "key3": "value3",
  "key4": "value4"  
}

Basically, I only need to copy the keys that are non-objects and non-arrays. In this example, key1, key2, key3 and key4

So here is what I did:

  var modObject = {}
  Object.keys(originalObj).forEach((child) => {

    // Copy non-objects and non-arrays
    if (typeof originalObj[child] !== 'object' && 
    !Array.isArray(modObject[child])) {
      modObject[child] = originalObj[child]
    }
  })
  
  // Change modObject and expect originalObj is updated to 'value3_modified' as well
  modObject['key3'] = 'value3_modified'
  console.log(originalObj['key3'])  // Outputs 'value3' - Key3 value was not modified
  console.log(modObject['key3'])  // Outputs 'value3_modified'

But the problem is, when I update modObject, its changes are not propagating to originalObj. I know, this is because I am copying it as a value.

So I tried top copy the entire object and just delete the ones that should not be there (arrays and object keys):

  // Copy the entire object
  modObject = originalObj

  Object.keys(modObject).forEach((child) => {

    // Delete objects and arrays - these should not be in the new object
    if (typeof modObject[child] === 'object' || 
    Array.isArray(modObject[child])) {
      delete modObject[child]
    }
  })

  // Update modObject and expect originalObj updated to 'value3_modified' as well
  modObject['key3'] = 'value3_modified'
  console.log(modObject)  // Outputs 'value3_modified'
  console.log(originalObj)  // // Outputs 'value3_modified' but object and array keys deleted in originalObj

But then, this time, my problem is since it is mutable, the keys that I deleted on the modObject also gets deleted in originalObj.

How can I achieve what I want:

Copy non-object and non-array keys from originalObj to modObject while maintaining its mutability?

Here is my fiddle: https://jsfiddle.net/keechan/hbdj9mcs/11/

Help! Thanks!

I am afraid this is actually impossible to achieve. At least I can't think of any way how. A variable is a pointer onto an object, not it's properties. So you can't let a variable only point to a subset of properties of an object. What is your actual use-case behind this? Would it be possible to use a function to access the object and then only return those keys you are interested in? — David Losert, Aug 18 '20 at 09:36
Primitives are copied, so changing them won't change them in the original object. Only reference types have the behavior that you want. — Yousaf, Aug 18 '20 at 10:12

score 0 · Answer 1 · answered Aug 18 '20 at 10:06

sorry i don't know what exactly you want, but i assume that you want to cloned original object and want to keep safe original object and modify cloned object.

Check if that works for you.

let modObject = JSON.parse(JSON.stringify(originalObj))

now do whatever with modObject(cloned) the originalObj dose not affect.

Stack Overflow: What is the most efficient way to deep clone an object in JavaScript?

Copy object properties maintaining mutability

1 Answers1