Update Javascript object property inside for loop

Question

I am trying to update a JS object with another object which seems trivial, but the value is not updating.

let sampleObj = {
  id: 1,
  name: 'Kelly'
}

let userData = [{
    students: [{
      id: 1,
      name: 'Sandra'
    }]
  },
  {
    students: [{
      id: 2,
      name: 'Jerome'
    }]
  }
]

for (let group of userData) {
  for (let student of group.students) {
    if (student.id === sampleObj.id) {
      console.log('updating student object')
      student = sampleObj

      // student = { ...sampleObj }     (another failed attempt)
      // userData[group].students[student] = sampleObj     (another failed attempt)
    }
  }
}

console.log('userData', userData)

It seems like, student is just some floating thing, unassociated to userData at the point where I'm trying to update its value. However, I can't figure out how to make the update or what I'm missing.

EDIT: The expected output is to replace the student object with sampleObj once its found.

What exactly is the expected output? Do you want to change the found student's name to the new one of the sampleObj? — CertainPerformance
– CertainPerformance, Commented Jan 18, 2019 at 0:32
I would like to replace the entire student object with sampleObj once its found — bruh
– bruh, Commented Jan 18, 2019 at 0:33
the var student only lives inside the inner for loop, so UserData is not affected from the assignment. — Jeff
– Jeff, Commented Jan 18, 2019 at 0:34
You need to replace the object at the same index in group.students. — Heretic Monkey
– Heretic Monkey, Commented Jan 18, 2019 at 0:35
I'd suggest - as connexo does, in his answer. - that you should use an Array.prototype.forEach() that way you have access to the array-element, the index and the array itself. — David Thomas
– David Thomas, Commented Jan 18, 2019 at 0:41

connexo · Accepted Answer · 2019-01-18 00:40:35Z

8

Use forEach(el, index) instead so you have the index available to do the update:

let sampleObj = {
  id: 1,
  name: 'Kelly'
}

let userData = [{
    students: [{
      id: 1,
      name: 'Sandra'
    }]
  },
  {
    students: [{
      id: 2,
      name: 'Jerome'
    }]
  }
]

userData.forEach((group, m) => {
  group.students.forEach((student, n) => {
    if (student.id === sampleObj.id) {
      console.log('updating student object')
      userData[m].students[n] = sampleObj
    }
  })
})

console.log('userData', userData)

answered Jan 18, 2019 at 0:40

connexo

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1 Comment

bruh Over a year ago

Alright so I was just using the wrong type of for loop. Thanks @connexo !

Kabir · Accepted Answer · 2019-01-18 04:27:28Z

6

Array item replacement does not work that way. Instead you can replace the properties of student object like this

for (let group of userData) {
  for (let student of group.students) {
    if (student.id === sampleObj.id) {
      console.log('updating student object')
      student = Object.assign(student, sampleObj);
    }
  }
}

It'll assign all the properties of samplObj to student object.

edited Jan 18, 2019 at 4:27

answered Jan 18, 2019 at 0:51

Kabir

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1 Comment

bruh Over a year ago

Worked like a charm. Just had to switch the order of parameters passed to Object.assign(student, sampleObj) - student first, sampleObj second . Thank you!!

Nishanth Matha · Accepted Answer · 2019-01-18 00:42:12Z

You're never updating the actual userData. To update it you should reference the userData object in the loop, Not the temporary variable which is used to iterate the object.

So try something like:

let sampleObj = { id: 1, name: 'Kelly' }

let userData = [
    {
    students: [
        { id: 1, name: 'Sandra' }
    ]
  },
  {
    students: [
        { id: 2, name: 'Jerome' }
    ]
  }
]
let j=0
for ( let group of userData ) {let i=0
    for ( let student of group.students ) { 
    if ( student.id === sampleObj.id ) {
        console.log('updating student object')
        userData[j].students[i] = sampleObj

      // student = { ...sampleObj }     (another failed attempt)
      // userData[group].students[student] = sampleObj     (another failed attempt)
    } i++;
  } j++;
}

console.log('userData', userData)

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