Given two javascript objects, how can I determine what field(s) were changed?

Question

Let's say I have two versions:

let oldObj = {
    name: "john",
    location: "usa",
    age: 43
}

let newObj = {
    name: "john",
    location: "usa",
    age: 44
}

In the case age changed.

Answer 1

Assuming you want to only track changes (of primitive values) and not additions/deletions, you can just iterate over the values from one object and do direct comparisons with corresponding values in the other object:

let oldObj = {
    name: "john",
    location: "usa",
    age: 43
}

let newObj = {
    name: "john",
    location: "usa",
    age: 44
}

let changes = Object
  .entries(oldObj)
  .filter(([key, value]) => value !== newObj[key])

console.log(changes)

If you don't care about the values, you can just filter the keys instead:

let oldObj = {
    name: "john",
    location: "usa",
    age: 43
}

let newObj = {
    name: "john",
    location: "usa",
    age: 44
}

let changes = Object
  .keys(oldObj)
  .filter(key => oldObj[key] !== newObj[key])

console.log(changes)

If you want to include additions/deletions, you can first determine the object with the larger amount of keys and do similarly to above:

let oldObj = {
    name: "john",
    location: "usa",
    age: 43
}

let newObj = {
    name: "john",
    location: "usa",
    age: 44,
    addition: 'test'
}

let [larger, smaller] = Object.keys(oldObj).length > Object.keys(newObj).length
  ? [oldObj, newObj]
  : [newObj, oldObj]

let changes = Object
  .entries(larger)
  .filter(([key, value]) => !smaller.hasOwnProperty(key) || value !== smaller[key])

console.log(changes)

Answer 2

Hooray for Lodash!

var changes = _.reduce(oldObj, function(result, value, key) {
    return _.isEqual(value, newObj[key]) ?
        result : result.concat(key);
}, []);
console.log(changes);

Answer 3

first thing you should do is to edit your question, and replace new with something else, because new is a keyword in javascript used with constructor functions to create an instance of that particular constructor function.

let old = {
    name: "john",
    location: "usa",
    age: 43
}

let _new = {
    name: "john",
    location: "usa",
    age: 44
}
for ( let j in old ) {
    if ( old[j] !== _new[j] ) {
        console.log(`${old[j]} is not ${_new[j]}`);
    }
}

the below code loops through the old object, the content of j will be the keys of the old object, the if statement uses old[j] to get the content of the j key ( same thing applies to _new[j] ), the if statement also checks if the content and the type of _new[j] and _old[j] is the same. If you don't care about the types , you should remove one of the equal signs in the if statement