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.

Bobby Hadz

bobbyhadz.com › blog › typescript-interface-default-values

How to set up TypeScript interface Default values | bobbyhadz

February 26, 2024 - Use the Pick utility type to only select the properties that have default values. Use the spread syntax to unpack the rest of the properties after the defaults when creating an object.

Discussions

Interface Default Values

This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.) This feature would agree with the rest of TypeScript's Design Goals. ... The best syntax would be , This suggests x default value is "y" and when x value is not given "y" is used · export interface ... More on github.com

github.com

July 12, 2023

Default value for interface property?

If you have Object.assign, or a polyfill for it, or something similar like $.extend in jQuery, then you can do:

const settings = Object.assign({}, defaults, options);

The es6.d.ts declaration looks like this:

assign<T, U>(target: T, source: U): T & U;
assign<T, U, V>(target: T, source1: U, source2: V): T & U & V;
assign<T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W;
assign(target: any, ...sources: any[]): any;

i.e. strongly-typed overloads up to 4 arguments. So you should get back an object whose type is the intersection of the arguments you passed in.

In short, this will perform defaulting for multiple properties in one go.

Videos