variable is undefined in typescript

Question

type User ={
    id: number;
    name?: string;
    nickname?: string;
}

type Info = {
    id: number;
    city?: string;
}

type SuperUser = User & Info;

let su:SuperUser;
su.id = 1; 

console.log(su);

This is a simple code. I did try the intersection types.Why console return me 'su' is undefined?

Where do you think you set a value for su? let su: SuperUser; only sets a type, the value is undefined. — jonrsharpe
– jonrsharpe, Commented Apr 19, 2020 at 9:24
@johnsharpe : sorry I did not pay attention to the type keyword. Comment deleted — Olivier Depriester
– Olivier Depriester, Commented Apr 19, 2020 at 9:26

Taghi Khavari · Accepted Answer · 2020-04-19 09:54:10Z

1

You need to assign some values to your variable first otherwise it would be undefined like this:

type User ={
    id: number;
    name?: string;
    nickname?: string;
}

type Info = {
    id: number;
    city?: string;
}

type SuperUser = User & Info;

let su:SuperUser = { // <-- First we assign some value
    id: 1
};
su.id = 2; // <-- and then we can use or change the value

console.log(su);

try on the playground

edited Apr 19, 2020 at 9:54

answered Apr 19, 2020 at 9:48

Taghi Khavari

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