In JavaScript objects are passed by reference, never by value. So:

var objDemo, objDemoBackup;
objDemo = {
    sub_1: "foo";
};
objDemoBackup = objDemo;
objDemo.sub_2 = "bar";
console.log(objDemoBackup.sub_2);   // "bar"

To get a copy, you must use a copy function. JavaScript doesn't have one natively but here is a clone implementation: How do I correctly clone a JavaScript object?

var objDemo, objDemoBackup;
objDemo = {
    sub_1: "foo";
};
objDemoBackup = clone(objDemo);
objDemo.sub_2 = "bar";
console.log(objDemoBackup.sub_2);   // undefined

To update easilly your array with a new value for specific id, use the map function :

rows = rows.map((row) => {
    if (row.id === new_item.id) {
        row = new_item;
    }

    return row;
});

There is always going to be a good debate on time vs space, however these days I've found using space is better for the long run.. Mathematics aside let look at a one practical approach to the problem using hashmaps, dictionaries, or associative array's whatever you feel like labeling the simple data structure..

    var marr2 = new Map(arr2.map(e => [e.id, e]));
    arr1.map(obj => marr2.has(obj.id) ? marr2.get(obj.id) : obj);

I like this approach because though you could argue with an array with low numbers you are wasting space because an inline approach like @Tushar approach performs indistinguishably close to this method. However I ran some tests and the graph shows how performant in ms both methods perform from n 0 - 1000. You can decide which method works best for you, for your situation but in my experience users don't care to much about small space but they do care about small speed.

Here is my performance test I ran for source of data

var n = 1000;
var graph = new Array();
for( var x = 0; x < n; x++){
  var arr1s = [...Array(x).keys()];
  var arr2s = arr1s.filter( e => Math.random() > .5);
  var arr1 = arr1s.map(e => {return {id: e, name: 'bill'}});
  var arr2 = arr2s.map(e => {return {id: e, name: 'larry'}});
  // Map 1
  performance.mark('p1s');
  var marr2 = new Map(arr2.map(e => [e.id, e]));
  arr1.map(obj => marr2.has(obj.id) ? marr2.get(obj.id) : obj);
  performance.mark('p1e');
  // Map 2
  performance.mark('p2s');
  arr1.map(obj => arr2.find(o => o.id === obj.id) || obj);
  performance.mark('p2e');
  graph.push({ x: x, r1: performance.measure('HashMap Method', 'p1s', 'p1e').duration, r2: performance.measure('Inner Find', 'p2s','p2e').duration});
}

If you don't want a new array, then .forEach, and add a new key with the value of the name, then delete name.

If you want a new array, then .map and you can for example create a new item object with the needed keys. This can be done in many ways.

I think youre overcomplicating it. You just need to update the property:

function changeEmail(user,email){
 user.email = email;
 return user;
}

So you can do:

changeEmail({email:"before"},"after");

If you wanna annoy your teacher with some ESnext:

const changeEmail = (user,email)=>({...user,email});
//(note this does shallow copy)
const user = changeEmail({email:"before"},"after");

And actually, i think this assignment isnt really useful. Why not simply:

user.email = "after";

Since you have an array of objects, you can set your active item to one of the objects, and that variable will be a reference to the object in the array. For example:

var array = [{a: 1, b: 2, c: 3}, {x: 1}, {x: 2}];
console.log(array[0]); // {a: 1, b: 2, c: 3}
var activeItem = array[0];
activeItem.a = 99;
console.log(array[0]); // {a: 99, b: 2, c: 3}

That way, writing the properties of activeItem writes the properties to the correct item in your array of objects.

You can use the map() function:

const updatedArray = sampleArray.map(item => item.id === 'hotel' ? {...item, id: 'hotel2'} : item); 

You can achieve this (not exactly what you want) if you wrap your object and change the modify function like this,

var wrapper = {};
wrapper.x = {a:1};
function modify(obj, key) {
    obj[key] = {b:2};
}
modify(wrapper, 'x');
console.log(wrapper.x); // {b:2}

On recent browsers with ECMAScript 2015, you can do:

Object.assign(skillet.person.name, { first: 'blah', last: 'ha'});

which will preserve any existing attribute not listed in the right object.

Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

[EDIT] With ES7, you can do even shorter, but it recreates the object (unlike Object.assign) and then adds some overhead if you care about performance. (comment thanks to Jamie Marshall)

{...skillet.person.name, ...{ first: 'blah', last: 'ha'}};

Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax