undefined element in array of array of type

Question

I have an array of users of type User = { id: number; name: string; floor: number }, where floor: number is a floor in the company where given user is working. I want to create an array building of N floors (number is constant), and every building[i] must contain a list of users (an array of User).

So it should look like this:

building = [
    1: [ [11, bob, 1], [42, jane, 1], [33, tom, 1] ],
    2: [ [14, amir, 2], [35, isaac, 2] ],
    ...
    N: [ [62, jack, N], [93, tobby, N], [21, elisa, N] ],
]

I try to implement idea in the next code:

type User = { id: number; name: string; floor: number }
building: Array<User[]>;
foo() {
    this.building = new Array<User[]>(3);
    ...
    for (let i = 0; i < users.length; i++) {
        if ( users[i].floor ) {
            this.building[users[i].floor].push(users[i]);
        }
    }
}

But when I run code there is an error: core.mjs:6485 ERROR TypeError: Cannot read property 'push' of undefined

What I missing?

you are not initializing your inner array you are just declaring something. — Judson Terrell
– Judson Terrell, Commented Jun 27, 2022 at 16:09
this.building = new Array<User[]>(3); does not create an array with three User[] arrays in it. It creates an array with three empty slots in it. — VLAZ
– VLAZ, Commented Jun 27, 2022 at 16:09

majusebetter · Accepted Answer · 2022-06-27 17:12:41Z

1

You have to initialize the arrays within the first array before you can push elements, like this:

class MyClass {
    type User = { id: number; name: string; floor: number }
    building: Array<User[]>;

    foo() {
        this.building = new Array<User[]>(3);
        // ...
        for (let i = 0; i < users.length; i++) {
            if (users[i].floor) {

                // Create the array if not already existing
                this.building[users[i].floor] ||= [];
                this.building[users[i].floor].push(users[i]);
            }
        }
    }
}

answered Jun 27, 2022 at 17:12

majusebetter

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