Keeping only certain properties in a JavaScript object

Question

I have an object. I would like to modify the object (not clone it) by removing all properties except for certain specific properties. For instance, if I started with this o

Answer 1

You can code your own implementation of _.pick, and use it according to your needs.

Having this snippet of code as the base for the following cases:

const myObj={
    p1:123,
    p2:321,
    p3:{p3_1:1231,p3_2:342},
    p4:'23423',
    //....
    p99:{p99_1:'sadf',p99_2:234},
    p100:3434
}
let properties= ['p1','p2', 'p3', 'p100'];

case 1:

You want a shallow copy (with references to vector values)

const myNewObj = properties.reduce((newObj,property)=>{newObj[property] = myObj[property]; return newObj}, {})
// if we modify the original vector value of 'p3' in `myObj` we will modify the copy as well:

myObj.p3.p3_1 = 99999999999;

console.log(myNewObj); // { p1: 123,​​​​​​​​​​  p2: 321,​​​​​​​​​​  p3: { p3_1: 99999999999, p3_2: 42 },​​​​​​​​​​  p100: 3434 }​​​​​

case 2:

You want a deep copy (losing references to vector values)

You can just use JSON utilities to that matter.

const myNewObj2 = properties.reduce((newObj,property)=>{newObj[property] = JSON.parse(JSON.stringify(myObj[property])); return newObj},{})

// no matter how hard you modify the original object, you will create a new independent object
myObj.p3.p3_1 = 99999999999;

console.log(myNewObj2) // { p1: 123, p2: 321, p3: { p3_1: 1231, p3_2: 342 }, p100: 3434 }​​​​​

Reusing case 2 with a function

You could implement a reducer to use it in different scenarios, like this one:

function reduceSelectedProps(origin, properties){
  return (newObj,property)=>{
    newObj[property] = JSON.parse(JSON.stringify(origin[property]));
     return newObj
  }
}

So you could have a more elegant reuse of it:

const myNewObj3 = properties.reduce(reduceSelectedProps(myObj, properties),{});
// no matter how hard you modify the original object, you will create a new independent object
myObj.p3.p3_1 = 99999999999;

console.log(myNewObj3) // { p1: 123, p2: 321, p3: { p3_1: 1231, p3_2: 342 }, p100: 3434 }​​​​​

disclaimers:

This is only an example that does not handle Date, Set, Map or function values inside the properties. To deal with all these cases (and many others), it needs a really complex function with checks on the prototypes and all that stuff. At this point, consider reusing the work of other developers using any library that could do it. Lodash?