Why *ngFor in Angular shows the new object first from the array of objects which is having .subscribe() method on in Ionic 3 - arrays

Question

I am new to Ionic 3 & Angular and ran into an issue with *ngFor and .subscribe() method. Please forgive me if it is an easy question. I tried to figure out the behaviour of http.get(..).map(..).subscribe() function when used along with *ngFor to maintain an array of objects and show the records to user using *ngFor on the .html template file but unable to know *ngFor's odd behaviour.
In my Ionic 3 App, I am getting the data using an API and storing it inside a component variable called "users" which is declared as below in component .ts file -
users: Array<any>;
I have below component function which gives me data for this users array -
addUser(count: number) {
this.http.get('https://randomuser.me/api/?results=' + count)
.map(data => data.json().results)
.subscribe(result => {
for (let val of result) {
this.users.push(val);
}
})
}
Initially, I get data for 10 users from the API using above component function by simply calling it inside my ngAfterViewInit() function like -
this.addUser(10);
This gives me 10 user record objects inside my users array which I show to the user using something like below in the view .html file -
<ion-card *ngFor="let user of users">
{{user.email}}
</ion-card>
At this time *ngFor puts the last array element at first in the view and shows the records in the descending order as the elements in the array starting from index 9 to 0.(LIFO order)
Now I start popping the last element from this users array using users.pop(); and push another element at the beginning at index 0 by shifting current elements using users.unshift(..) in below function and calling it as addNewUser(1); -
addNewUser(count: number) {
this.http.get('https://randomuser.me/api/?results=' + count)
.map(data => data.json().results)
.subscribe(result => {
for (let val of result) {
this.users.unshift(val);
}
})
}
At this moment, if we consider the first array which had 10 elements, the last object at index 9 had been removed and another element at index 0 has been added making the previous elements on index 0-8 to shift to index 1-9.
On doing so, my view gets updated which has *ngFor and surprisingly this time it shows the first element at first place which is actually on index 0 which is the one I recently put. This is opposite to the order earlier followed by *ngFor to render elements on the screen.
Why *ngFor in Ionic 3 view shows the recently inserted object element first from the array of objects which is dependent on the subscribe method .subscribe() method. I am really confused about this.
I really need to clear the concept of *ngFor and subscribe(). Please help me.
Note : The API mentioned above is publicly accessible for testing and you may call it to check the response structure if required.
Pasting a sample API response below on calling https://randomuser.me/api/?results=1 -
{
"results": [
{
"gender": "male",
"name": {
"title": "mr",
"first": "daniel",
"last": "stoll"
},
"location": {
"street": "9719 tannenweg",
"city": "cottbus/chosebuz",
"state": "bremen",
"postcode": 81443
},
"email": "daniel.stoll#example.com",
"login": {
"username": "greenleopard994",
"password": "chat",
"salt": "OUjisBdQ",
"md5": "8148d51998f3fef835a5f3979218c181",
"sha1": "131ae09d045b345efe36a330bf17a450b76f7de3",
"sha256": "94c3a362b5f516d0fb1d4e9dbb632d32d57b8886d5cc7bf0d5cedc99e7d55219"
},
"dob": "1957-04-26 22:07:14",
"registered": "2002-04-29 10:57:34",
"phone": "0556-4348870",
"cell": "0172-5116200",
"id": {
"name": "",
"value": null
},
"picture": {
"large": "https://randomuser.me/api/portraits/men/14.jpg",
"medium": "https://randomuser.me/api/portraits/med/men/14.jpg",
"thumbnail": "https://randomuser.me/api/portraits/thumb/men/14.jpg"
},
"nat": "DE"
}
],
"info": {
"seed": "8fd4afe85884c767",
"results": 1,
"page": 1,
"version": "1.1"
}
}
Refer this example showing my issue.

Answer 1

If you have a sorting issue with indexing and you think it's related.. you could work around the issue by assigning an index:
*ngFor="let user of users; let i = index"
and then reference the direct index value
users[i]

Answer 2

You should make a copy of that array. Editing array elements while looping them can lead to unexpected behaviour.