stackoverflow.com › questions › 35074365 › typescript-interface-default-values

Can I tell the interface to default the properties I don't supply to null? What would let me do this

No. You cannot provide default values for interfaces or type aliases as they are compile time only and default values need runtime support

Alternative

But values that are not specified default to undefined in JavaScript runtimes. So you can mark them as optional:

interface IX {
  a: string,
  b?: any,
  c?: AnotherType
}

And now when you create it you only need to provide a:

let x: IX = {
    a: 'abc'
};

You can provide the values as needed:

x.a = 'xyz'
x.b = 123
x.c = new AnotherType()

Answer from basarat on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 35074365 › typescript-interface-default-values

Typescript interface default values - Stack Overflow

Alternative

But values that are not specified default to undefined in JavaScript runtimes. So you can mark them as optional:

interface IX {
  a: string,
  b?: any,
  c?: AnotherType
}

And now when you create it you only need to provide a:

let x: IX = {
    a: 'abc'
};

You can provide the values as needed:

x.a = 'xyz'
x.b = 123
x.c = new AnotherType()

2 of 16

157

You can't set default values in an interface, but you can accomplish what you want to do by using optional properties:

Simply change the interface to:

interface IX {
    a: string,
    b?: any,
    c?: AnotherType
}

You can then do:

let x: IX = {
    a: 'abc'
}

And use your init function to assign default values to x.b and x.c if those properies are not set.

reddit.com › r/typescript › how do i pass a default value if object is undefined

r/typescript on Reddit: how do I pass a default value if object is undefined

March 30, 2022 -

I have a class like this but I want to initialize a default value for Id and email when there is no data. How do I do that? As you see, I want to set the default value if I pass new Recipient ()

const user = this.options.find(x=>x.id == 1);

return user? new Recipient (id:user.id, email:user.email): new Recipient ();

export class Recipient {
    id: number;
    email?: string;
    constructor(data?: Partial<Recipient>) {
        Object.assign(this, data);
    }
}

=======================Update======================

eventually I chose one of two options

export class Recipient {
    id: number;
    email?: string;
    constructor(data?: Partial<Recipient>) {
        this.id = data?.id || 0;
        this.email = data?.email || null;
    }
}
export class Recipient {
    id: number;
    email?: string;
    constructor(data?: Partial<Recipient>) {
        Object.assign(this,{id:0, email :null}, data);
    }
}

Top answer

1 of 2

I don't really understand why you loosely Type something when the whol point of TS is to use strict typing. But assuming you have a genuine use case here you can assign defulat values when declaring properties inside the class. Like so export class Recipient { id: number = 1; email?: string = somedfault@email.com; } Here 1 will be assigned a default value to number and same goes for the email ass well. And if you pass in a value in the constructor during the instantiatio of the class that will override the id and the email. Do remeber to check and assign values in the constructor toneaxh of these properties else they will always default to the above values.👍

2 of 2

The Recipient type export class Recipient { id: number; email?: string; constructor(data?: Partial) { this.id = data?.id || 0; this.email = data?.email || null; } } allows to create the following combinations: id | email | 0 | undefined | 0 | null | number | undefined | number | null | number | string If any of those combinations are invalid in your domain, then it's a possible bug waiting to happen. Not to count, you need to handle both null and undefined. With strict enabled, two combinations are gone because this.email = data?.email || null; wouldn't compile . Then, you'd be left with something like: id | email | 0 | undefined | number | undefined | number | string Do they all make sense in your domain? Could you misuse any of them? I could imagine that a Recipient with id 0 represents one to save in the database: const saveRecipient = (recipient: Recipient) => // ... The issue is that you can now call saveRecipient with an id 0 and, if you forget to check what's the value of the id, the database would either raise an error or do something silly (eg, actually use id 0). Personally, I like to be really strict with types so that I don't have to remember to perform checks—the compiler reminds me: type PersistedRecipient = { id: number, email: string } type NewRecipient = { email: string } // or Omit const saveRecipient = (recipient: Recipient) => // ... Notice, I cannot call saveRecipient with a value of type PersistedRecipient.

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