根据字段将打字稿映射对象映射到具有新对象的数组

Question

I have an object with some concrete fields:

 const obj1: Obj = {  
    hello: true, 
    second: false, 
    third: 311,  
    fifty: 50  
    .....  
}  

I need to map some fields from this object to concrete number, (and use it value) and return array with it:

interface ArrayItem {
    index: number;
    visible: boolean;
}

// hello should be mapped to 3, and visible - 'true'
// result is:
const arr: ArrayItem[] = [{index: 3, visible: true}]

For concrete field. eg if obj1, has field "hello", result will be:

[  
    {
     index: 3  
     visible: (value of 'hello', so -> true)  
    }  
]  

More examples:

 const obj1: Obj = {
    hello: true,
    second: false,
    third: 311,
    fifty: 50
    .....
}

// fields, which i looking:

const mapped = {
    hello: 3, // index value
    second: 5 // index value
}

output:

[
    {
     index: 3
     visible: (value of 'hello', so -> true)
    }
    {
     index: 5
     visible: (value of 'second', so -> false)
    }
]

Answer 1

Let's declare type definition of Obj

type Obj = { [key: string]: boolean };

Object with many key, and bool on all values.

Array items look like this:

interface ArrayItem {
    index: number;
    visible: boolean;
}

But we can get some more knowledge about types based on values from Obj . To declare that let's write InterfaceParser generic.

type InterfaceParser<T extends Obj> = {
  index: T['hello'] extends true ? 3 : number;
  visible: boolean;
}

T extends Obj means that input type must be at least Obj . extends true ? 3 : number extends true ? 3 : number means that when hello key is true the type of index is not number is 3 .

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