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.

Mimo

mimo.org › glossary › typescript › interface

TypeScript Interface: Syntax, Usage, and Examples

Interfaces do not support default values directly, but you can use a factory function.

Discussions

Interface Default Values

Suggestion This will be a very useful feature If we could provide default values for interfaces , In Kotlin you can already do this using interface Y { val x : Int get() = 5 } There's a stackov... More on github.com

github.com

July 12, 2023

Defaulting Unspecified Interface Properties to Null in TypeScript - Ask a Question - TestMu AI Community

I have the following TypeScript interface: interface IX { a: string; b: any; c: AnotherType; } I declare a variable of this type and initialize all the properties with default values: let x: IX = { a: … More on community.testmuai.com

community.testmuai.com

July 25, 2024

Is there a way to have a default value for an object argument passed to a function?

Destructuring with default value is more readable in most cases. If you don’t want to destructure everything you can do: function order(orderInfo: OrderInfo) { const {onSale = true} = orderInfo; } Then you don’t lose the original object More on reddit.com

r/typescript

January 7, 2026

Interface hint helper showing values that will be used defaultly ?

Interfaces, i think at least, is intended for example to define what properties and methods must be defined, their types and their eventual return values. It is then up to each class that implementes the interface to handle exactly how. The fictive Interface EmailSendingInterface does for example not dictate what api to call or how error handling should be implemented, but rather what methods and params other programs will use to utilize a class that implements the EmailSendingInterface.

Fictive class MailgunService might implememt the EmailSendingInterface interface, but the interface does not all care how the class sends emails (or if it even does). It just says hey, if they claim to conform to me they will have a SendEmail( sender: string, content: string) method. MailgunService might implement that method using mailgun api, but MockSender might implement that by console logging the parameters and returning an OK to the calling class.

Does it make it clearer why interfaces do not dictate values and implementations but only conformity rules for the actual implementating classes?

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