在 Typescript 中创建接口而不指定属性名称

Question

I remember there is a way to create, inside an interface, a field without specifying its name, something like this:

export interface myInterface{
 [propName:string]:string;
}

If I remember well, that sinthax means I can create a field with whatever name I want, something like this:

ob:myInterface = {customName: "value"}

What I'm trying to do now is to add a new field to that interface, with a specific name, something like this:

export interface myInterface{
 [propName:string]:string;
 secondProperties: boolean;
}

When I try the code above I get this error:

Property 'secondProperties' of type 'boolean' is not assignable to string index type 'string'

What is my error?

Answer 1

You need to define all possible types for [propName: string] So you need to do it in this way

export interface myInterface{
  secondProperties: boolean
  [propName:string]: string | boolean;
}

Answer 2

I never found a good solution but the problem arises from this.

You are trying to force a property to be of type boolean & string , which equals to never .

type myInterface = {
    [propName:string]: string;
} & {
    secondProperties: boolean;
};

const obj: myInterface = {
    secondProperties: true,
};

playground

---

Thanks to @LaytonGB for hinting, that | can be used to make almost the type we wanted.

type myInterface = {
    [propName:string]: string;
} | {
    secondProperties: boolean;
};

const obj: myInterface = {
    secondProperties: 'string' || true,
    otherProps: 'string only', // booleans will result in error.
};

Answer 3

Because you have defined any string-named-property of your objects to have a value of string, you can only give secondProperties a string value, because secondProperties itself is a string.

Alternatively consider this:

interface myInterface {
  [propName: string]: string | boolean;
  ["secondProperties"]: boolean;
}

Because secondProperties is a string, and its value returns a boolean, it is not throwing errors.