如何定义选择嵌套属性而不丢失 Typescript 中的类型的 af 函数

Question

i am looking for a way to select a nested property in a object of objects without loosing the type information of the given property using Typescript.

This could be the input for the function:

var fields = {email: {$: 'test'}, password: {$: 1}};

I would like to have a function that transforms this fields object into

var result = {email: 'test', password: 1}

Without loosing the type information on each fields. The email stays a string and password stays a number.

The real use case is to pick the value field out of some form data object in order serilize it into JSON.

Answer 1

You can do this in Typescript 2.8 (unreleased at the time of writing, will be released in March 2018). This involves using conditional types and their associated inference behavior:

type Unpacked<T> =
    T extends { $: infer U } ? U : // if T has $ then we extarct the type of $
    T;

var fields = {email: {$: 'test'}, password: {$: 1}};

function unpack<T>(obj: T) : { [P in keyof T] : Unpacked<T[P]>} {
    return  <any>null; // dummy implementation
}

var r = unpack(fields) // Will be typed as { email: string, password: number }