Typescript Definition - How to define an array property in object that accept certain strings

You have a some options depending on your exact situation:

Enum: Provides flexible, type-safe definition of your values that allow for run-time look-ups and reverse look-ups. This is likely your preferred solution:

enum Channel {
    Email = 'email', // if you prefer email = 'email', etc. that's also very doable
    WhatsApp = 'whatsApp',
    WeChat = 'weChat',
    Skype = 'skype',
    Line = 'line',
    SMS = 'sms',
}

class User {
   constructor(private name: string, private communicationPreferences: Channel[]) { };
}

const john = new User('john', [Channel.SMS]); // {name: 'john', communicationPreferences: ['sms']);

// example of reverse look-up
const channel = 'skype'; // from some unverified source
if (Channel[channel] === undefined) {
   // handle error case at run-time
}

Union Type: If you wanna be quick and dirty about it, you can use a string literal union type. This would provide compile-time only safety, or if you have a very extreme case where you want to limit the (very small) overhead of an Enum:

type Channel = 'email'| 'whatsApp'| 'weChat' | 'skype' | 'line'| 'sms';

interface User {
   name: string;
   communicationPreferences: Channel[];
}

const john: User = {
    name: 'john',
    communicationPreferences: ['telegraph']; // would fail to compile
}

Index Type: Your last option would be combining the keyof and typeof operators to produce a dynamic union type. This might be useful if your communication channel options are coming from an external JSON/JS file, especially if it may change:

// Some example object you're getting

const channels = {
   skype: { ... },
   sms: { ... },
   line: { ... },
   // and so on
}

// in your script
type Channel = keyof typeof channels; // "skype" | "sms" | "line" | ...

In your situation, you might also be interested in the Set class; it provides a distinct (only one of each value allowed) collection:

// define Channel type, from options above

class User {
    private communicationPreferences: Set<Channel>;

    constructor(private name: string, channels: Channel[]) {
        this.communicatonPreferences = new Set<Channel>(channels);
    }

    public serialize() {
        return {
            name: this.name,
            communicationPreferences: Array.from(this.communicationPreferences)
        }
    }
}