What is the most efficient way to copy some properties from an object in JavaScript?

Question

Say, I have an object:

const user = {_id: 1234, firstName: 'John', lastName: 'Smith'}

I want to create another object without the _id key:

const newUser = {firstName: 'John', lastName: 'Smith'}

I am using this:

const newUser = Object.assign({}, {firstName: user.firstName, lastName: user.lastName})

Is there a better way to do this?

Answer 1

You can achieve it with a form of destructuring:

const user = { _id: 1234, firstName: 'John', lastName: 'Smith' };
const { _id, ...newUser } = user;
console.debug(newUser); 

However, at the time of writing this answer, the spread (...) syntax is still at the ECMAScript proposal stage (stage 3), so it may not be universally available in practice. You may use it with a "transpilation" layer such as Babel.

Answer 2

Do it with Array#reduce method with Object.keys method.

const user = {
  _id: 1234,
  fistName: 'John',
  lastName: 'Smith'
};

var res = Object.keys(user).reduce(function(obj, k) {
  if (k != '_id') obj[k] = user[k];
  return obj;
}, {});

console.log(res);

Answer 3

You are taking a shallow copy twice: once with the object literal, and again with Object.assign. So just use the first of the two:

const newUser = {firstName: user.firstName, lastName: user.lastName};