Insert a new object in Array of object

Question

I have an Array List.

dataList = []

I want to insert object in array, So I tried this,

dataList.concat([{"name":"BOB", "value":"1"}])
// [{"name":"BOB", "value":"1"}]

but when I insert 2nd object in the same array.

dataList.concat([{"name":"Joe", "value":"2"}])
// [{"name":"Joe", "value":"2"},{"name":"Joe", "value":"2"}]

after inserting second array it replaces first object also. where am I wrong?, Please help.

To insert a new value in array, use `Array.push`. `Array.concat` returns a new array — Rajesh, Jan 03 '18 at 05:37
I have tried with push method also . It does the same in my case. — Amir, Jan 03 '18 at 05:38
Do you have a variable that holds object or you have inline object? — Rajesh, Jan 03 '18 at 05:39
i have inline array of object. dataList = [{"name":"BOB", "value":"1"}] newDataList = [{"name":"Joe", "value":"2"}] dataList.concat(newDataList) // [{"name":"Joe", "value":"2"},{"name":"Joe", "value":"2"}] — Amir, Jan 03 '18 at 05:40
Looking at the behavior, you must have it in a variable. The issue is, objects are assigned using reference. So changing 1 variable will update value in all variables holding this reference — Rajesh, Jan 03 '18 at 05:41
Your example doesn't do what you say. `concat` doesn't merge arrays in place, it returns a new array. — Teemu, Jan 03 '18 at 05:42

score 0 · Answer 1 · answered Jan 03 '18 at 05:49

0

Simply use push function

var dataList = [];
dataList.push({
  "name": "BOB",
  "value": "1"
});
//Pushing second object
dataList.push({
  "name": "Joe",
  "value": "2"
})
console.log(dataList)

answered Jan 03 '18 at 05:49

brk

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If this is the ideal answer, should we not close it as dupe? – Rajesh Jan 03 '18 at 05:58
I just realized, OP has confirmed using `.push`. 2nd comment under question. Please check – Rajesh Jan 03 '18 at 06:06

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

To push a value in an array, you will have to use array.push. array.concat does not adds new item. it merges 2 array and return a third array. So your code:

dataList.concat([{"name":"BOB", "value":"1"}])

does not do anything.

var dataList = []
dataList.concat([{"name":"BOB", "value":"1"}])

console.log(dataList)

As per the behavior, your code should look something like this:

var dataList = []
var obj = [{"name":"BOB", "value":"1"}];
dataList = dataList.concat(obj)
obj[0].value = 2;
dataList = dataList.concat(obj)
console.log(dataList)

The reason both the objects are being affected is because, objects are copied/assigned using reference. So a variable will hold a memory location and any changes to it will be reflected to this location. So if you have more than 1 variable holding this reference, it will also get updated.

So how should you achieve this?

var dataList = []
var obj = {"name":"BOB", "value":"1"};
dataList = dataList.concat(obj)
var obj2 = Object.assign({}, obj)
obj2.value = 2;
dataList = dataList.concat(obj2)
console.log(dataList)

But still using Array.concat is wrong. It is not intended to be used like this.

var dataList = []
var obj = {"name":"BOB", "value":"1"};
dataList.push(obj)
var obj2 = Object.assign({}, obj)
obj2.value = 2;
dataList.push(obj2)
console.log(dataList)

References:

**Note:** Not closing as dupe as OP has confirmed he/she has tried using `.push` but it did not solve the issue. So thought of explaining what and why. — Rajesh, Jan 03 '18 at 06:05

Insert a new object in Array of object

2 Answers2

References: