Add key value pair to all objects in array

I would be a little cautious with some of the answers presented in here, the following examples outputs differently according with the approach:

const list = [ { a : 'a', b : 'b' } , { a : 'a2' , b : 'b2' }]

console.log(list.map(item => item.c = 'c'))
// [ 'c', 'c' ]

console.log(list.map(item => {item.c = 'c'; return item;}))
// [ { a: 'a', b: 'b', c: 'c' }, { a: 'a2', b: 'b2', c: 'c' } ]

console.log(list.map(item => Object.assign({}, item, { c : 'c'})))
// [ { a: 'a', b: 'b', c: 'c' }, { a: 'a2', b: 'b2', c: 'c' } ]

I have used node v8.10.0 to test the above pieces of code.

You can do this with map()

var arrOfObj = [{
  name: 'eve'
}, {
  name: 'john'
}, {
  name: 'jane'
}];

var result = arrOfObj.map(function(o) {
  o.isActive = true;
  return o;
})

console.log(result)

If you want to keep original array you can clone objects with Object.assign()

var arrOfObj = [{
  name: 'eve'
}, {
  name: 'john'
}, {
  name: 'jane'
}];

var result = arrOfObj.map(function(el) {
  var o = Object.assign({}, el);
  o.isActive = true;
  return o;
})

console.log(arrOfObj);
console.log(result);

The map() function is a best choice for this case

tl;dr - Do this:

const newArr = [
  {name: 'eve'},
  {name: 'john'},
  {name: 'jane'}
].map(v => ({...v, isActive: true}))

The map() function won't modify the initial array, but creates a new one. This is also a good practice to keep initial array unmodified.

Alternatives:

const initialArr = [
      {name: 'eve'},
      {name: 'john'},
      {name: 'jane'}
    ]

const newArr1 = initialArr.map(v => ({...v, isActive: true}))
const newArr2 = initialArr.map(v => Object.assign(v, {isActive: true}))

// Results of newArr1 and newArr2 are the same

Add a key value pair conditionally

const arr = [{value: 1}, {value: 1}, {value: 2}]   
const newArr1 = arr.map(v => ({...v, isActive: v.value > 1}))

What if I don't want to add new field at all if the condition is false?

const arr = [{value: 1}, {value: 1}, {value: 2}]   
const newArr = arr.map(v => {
  return v.value > 1 ? {...v, isActive: true} : v 
})

Adding WITH modification of the initial array

const initialArr = [{a: 1}, {b: 2}]
initialArr.forEach(v => {v.isActive = true;});

This is probably not a best idea, but in a real life sometimes it's the only way.

Questions

• Should I use a spread operator(...), or Object.assign and what's the difference?

Personally I prefer to use spread operator, because I think it uses much wider in modern web community (especially react's developers love it). But you can check the difference yourself: link(a bit opinionated and old, but still)

• Can I use function keyword instead of =>?

Sure you can. The fat arrow (=>) functions play a bit different with this, but it's not so important for this particular case. But fat arrows function shorter and sometimes plays better as a callbacks. Therefore the usage of fat arrow functions is more modern approach.

• What Actually happens inside map function: .map(v => ({...v, isActive: true})?

Map function iterates by array's elements and apply callback function for each of them. That callback function should return something that will become an element of a new array. We tell to the .map() function following: take current value(v which is an object), take all key-value pairs away from v andput it inside a new object({...v}), but also add property isActive and set it to true ({...v, isActive: true}) and then return the result. Btw, if original object contains isActive filed it will be overwritten. Object.assign works in a similar way.

• Can I add more then one field at a time

Yes.

[{value: 1}, {value: 1}, {value: 2}].map(v => ({...v, isActive: true, howAreYou: 'good'}))

• What I should not do inside .map() method

You shouldn't do any side effects[link 1, link 2], but apparently you can.

Also be noticed that map() iterates over each element of the array and apply function for each of them. So if you do some heavy stuff inside, you might be slow. This (a bit hacky) solution might be more productive in some cases (but I don't think you should apply it more then once in a lifetime).

• Can I extract map's callback to a separate function?

Sure you can.

const arr = [{value: 1}, {value: 1}, {value: 2}]   
const newArr = arr.map(addIsActive)

function addIsActive(v) {
    return {...v, isActive: true}
}

• What's wrong with old good for loop?

Nothing is wrong with for, you can still use it, it's just an old-school approach which is more verbose, less safe and mutate the initial array. But you can try:

const arr = [{a: 1}, {b: 2}]
for (let i = 0; i < arr.length; i++) {
 arr[i].isActive = true
}

• What also i should learn

It would be smart to learn well following methods map(), filter(), reduce(), forEach(), and find(). These methods can solve 80% of what you usually want to do with arrays.

You may also try this:

arrOfObj.forEach(function(item){item.isActive = true;});

console.log(arrOfObj);