Add key value (object) pair to all objects in array

Question

My question is related to Add key value pair to all objects in array

However, the solutions don't work when I try to assign an object instead of a string, int etc.

I tried to create the new key inside map function, but it only works with non-object variables.

This works

arrObjects.map(item => item.newKey = 'xxx')

This doesn't

var obj = { a: 'a', b: 'b', c: 'c' }
arrObjects.map(item => item.newKey = obj)

Output:

var arrOfObj = [{
  name: 'eve'
}, {
  name: 'john'
}, {
  name: 'jane'
}];
var obj = {
  a: 'a',
  b: 'b',
  c: 'c'
}
arrOfObj.map(item => item.newKey = obj);

console.log(arrOfObj);

.as-console-wrapper { max-height: 100% !important; top: 0; }

What is arrObjects? Can you post the rest of your code please? — Jack Bashford
– Jack Bashford, Commented Feb 9, 2019 at 20:34
Also, you might clarify what you mean by doesn't work. What are you expecting? What is happening instead? — Mark
– Mark, Commented Feb 9, 2019 at 20:35
You shouldn't mutate the original array when using map. Use: arrObjects.map(item => ({...item, newKey: {...obj}})); instead — adiga
– adiga, Commented Feb 9, 2019 at 20:46

Code Maniac · Accepted Answer · 2019-02-09 20:44:24Z

3

You need to create a copy of object. By default object is assigned as reference. here ... is used to create a shallow copy

var arrOfObj = [{name: 'eve'}, {name: 'john'}, { name: 'jane'}];
var obj = { a: 'a', b: 'b', c: 'c'}

arrOfObj.forEach(item => (item.newKey = {...obj}));

console.log(arrOfObj);

.as-console-wrapper { max-height: 100% !important; top: 0; }

You can see some of use case here

edited Feb 9, 2019 at 20:44

answered Feb 9, 2019 at 20:38

Code Maniac

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Jack Bashford · Accepted Answer · 2019-02-09 20:49:45Z

3

You need to use ... (spread operator)

var arrOfObj = [{ name: 'eve' }, { name: 'john' }, { name: 'jane' }];
var obj = { a: 'a', b: 'b', c: 'c' };
arrOfObj.forEach(item => item.newKey = {...obj});

console.log(arrOfObj);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Alternatively, use JSON:

var arrOfObj = [{ name: 'eve' }, { name: 'john' }, { name: 'jane' }];
var obj = { a: 'a', b: 'b', c: 'c' };
arrOfObj.forEach(item => item.newKey = JSON.parse(JSON.stringify(obj)));

console.log(arrOfObj);

.as-console-wrapper { max-height: 100% !important; top: 0; }

edited Feb 9, 2019 at 20:49

answered Feb 9, 2019 at 20:39

Jack Bashford

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Comments

Shidersz · Accepted Answer · 2019-02-09 20:52:10Z

2

One alternative is to use Object.assign(). Remenber that objects are copied by the value of the reference, that was your problem, instead of a new object for every newKey, you had multiple references to the same object.

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

var obj = { a: 'a', b: 'b', c: 'c' };

arrOfObj.map(item => item.newKey = Object.assign({}, obj));
console.log(arrOfObj);

// After some modification.
arrOfObj[0].newKey.a = "XXX"
console.log(arrOfObj);

edited Feb 9, 2019 at 20:52

answered Feb 9, 2019 at 20:44

Shidersz

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2 Comments

Madara's Ghost Over a year ago

Note that Object.create() doesn't do exactly what you say it does. Object.create() creates a new object and sets the prototype to be the argument you pass to it. This is not the same as creating a copy of the object. For that matter, making a change to obj will still result in a perceived change in the created objects, even though they don't share a reference.

Shidersz Over a year ago

@MadaraUchiha you were right, thank you, I changed to Object.assign() instead

Alen.Toma · Accepted Answer · 2019-02-09 20:43:35Z

0

You have to clone the item, you are adding the same object to multiple property.

See below how it should be.

var arrOfObj = [{
  name: 'eve'
}, {
  name: 'john'
}, {
  name: 'jane'
}];
var obj = {
  a: "a",
  b: "b",
  c: "c"
}
arrOfObj.map(item => 
// clone the item 
item.newKey = JSON.parse(JSON.stringify(obj))
);

console.log(arrOfObj);

answered Feb 9, 2019 at 20:43

Alen.Toma

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