What is the cause of the error message "property 'map' is undefined"?

I am currently working on a service that looks like this:

public getUsers() {
  return this.httpClient.get(environment.BASE_URL + `api/all`);
}

In my component, I have implemented the ngx-bootstrap table to display the user data.

import { Component, OnInit } from "@angular/core";
import { UserService } from "./service.service";
import { IUser } from "./users.model";

@Component({
  selector: "app-users",
  templateUrl: "./users.component.html",
  styleUrls: ["./users.component.css"]
})
export class UsersComponent implements OnInit {
  allUsers: IUser[];
  page = 1;
  pageSize = 4;
  collectionSize: any;

  constructor(private userService: UserService) {}

  ngOnInit() {
    this.userService.getUsers().subscribe((data: any[]) => {
      this.collectionSize = data.length;
      this.allUsers = data;
    });
  }

  get users(): IUser[] {
    return this.allUsers.map((user, i) => ({ id: i + 1, ...user })).slice((this.page - 1) * this.pageSize, (this.page - 1) * this.pageSize + this.pageSize);
  }
}

Despite displaying all the data in the table on the DOM, I keep encountering the error message

TypeError: Cannot read property 'map' of undefined
.

I attempted to resolve this by using .pipe(), but then I started receiving the error:

TypeError: Cannot read property 'pipe' of undefined
.

It's puzzling to me as the data is being shown correctly in the UI, yet this error persists in the console.

Answer №1

The reason behind this issue is that in Angular, getters are reevaluated whenever there is a change detection event. Since the allUsers array is populated asynchronously, there can be instances when it is undefined, leading to the error message you encountered.

An effective solution would be to initialize allUsers as an empty array initially, ensuring it will never be undefined:

this.allUsers = [];

Keep in mind that the above code block will execute every time change detection is triggered:

return this.allUsers.map((user, i) => ({ id: i + 1, ...user })).slice((this.page - 1) * this.pageSize, (this.page - 1) * this.pageSize + this.pageSize);
}

Consider whether this repetitive execution is necessary. If not, you can enhance performance by avoiding the use of a getter for users. Instead, create a global variable like allUsers, and perform the mapping within the callback of this.userService.getUsers().

Answer №2

Finishing up maryrio7's response

Your API call is being made asynchronously, which means it can take a while to fetch the data. While the call is still processing, the value of this.allUsers remains unset, resulting in it being undefined. This is why you encounter the error message.

Once the asynchronous operation completes, this.allUsers is populated with data and change detection kicks in on the reference. This is why all the data suddenly appears in the table. However, keep in mind that the error was triggered earlier in the process.

Answer №3

Make sure to declare the allUsers variable before assigning any data:

this.allUsers = [];

Answer №4

For a more concise solution, you can avoid adding an extra line by initializing the allUsers array when it is defined. Simply change

allUsers: IUser[]; 

to

allUsers: IUser[]=[];

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