Using Javascript .find() seems to be doing some weird coercion I can't seem to pinpoint

Question

I'm trying to collect all the objects from DATA and compare to schemesFromPOI. If the name is AMERICAN_EXPRESS I want to change it to AMEX in the scheme object only, preserving the DATA object.

const DATA =
    [
      {
        "name": "MASTERCARD",
        "id": "MASTERCARD123"
      },
      {
        "name": "VISA",
        "id": "VISA456"
      },
      {
        "name": "AMERICAN_EXPRESS",
        "id": "AMEX789"
      }
    ];

const schemesFromPOI = [
  { dpaId: 'DPA1', name: 'MASTERCARD' },
  { dpaId: 'DPA2', name: 'VISA' },
  { name: 'AMERICAN_EXPRESS' }
];

let config = [];

schemesFromPOI.forEach(function (schemeFromPOI) {

  let scheme = DATA.find(function (element) { return element.name === schemeFromPOI.name; });

  if(scheme) {
    if (scheme.name === "AMERICAN_EXPRESS") {
      console.log("Before updating scheme.name property: ", DATA);
      scheme.name = 'AMEX';
      console.log("After updating scheme.name property: ", DATA);
    }

    config.push(scheme);
  }

});

However, when I change scheme.name, the DATA object is updated as well. Here is the result of the above console outputs:

Before updating scheme.name property: "[{'name':'MASTERCARD','id':'MASTERCARD123'},{'name':'VISA','id':'VISA456'},{'name':'AMERICAN_EXPRESS','id':'AMEX789'}]"
After updating scheme.name property: "[{'name':'MASTERCARD','id':'MASTERCARD123'},{'name':'VISA','id':'VISA456'},{'name':'AMEX','id':'AMEX789'}]"

Clearly the DATA object is being updated, not just the scheme object. Why is this coercion happening? Does it have to do with the .find() method?

Answer 1

It happens because arrays are reference types. So when you are editing scheme.name = 'AMEX';, then it means that scheme.name will be changed and an object which was found in DATA.

If you want to create a brand new object, then use spread ... operator :

const data = [
    { name: "John", age: "22", weight: "60", height: "180cm" },
    { name: "Emma", age: "25", weight: "55", height: "160cm" }
];

let obj = data.find(f=> f.name =='John');

let brandNewObject = {...obj};

An example:

const data = [
        { name: "John", age: "22", weight: "60", height: "180cm" },
        { name: "Emma", age: "25", weight: "55", height: "160cm" }
    ];
    
    let obj = data.find(f=> f.name =='John');        
    let brandNewObject = {...obj};        
    brandNewObject .name = "Joseph";
    
    console.log(`obj`, obj);
    console.log(`brandNewObject `, brandNewObject );