TypeScript mapped types: from list of both objects and strings to a key-value object

Question

I'm trying to write a mapped type that takes a list which can contain both objects ("definitions") and strings. String values are limited to a set of strings that refer to existing hard coded definitions. The resulting type should be an object where the keys are collected from that input list;

• for list elements of type string, use that string
• for list elements of type object, use the name property from that object

Here is my current code:

type CustomDefinition = {
    name: string;
    units: string | null;
};

const predefinitions = {
    'AUTOPILOT_AIRSPEED_ACQUISITION': {
        name: 'AUTOPILOT_AIRSPEED_ACQUISITION',
        units: 'Bool',
        settable: false,
    },
    'AUTOPILOT_AIRSPEED_HOLD': {
        name: 'AUTOPILOT_AIRSPEED_HOLD',
        units: 'Bool',
        settable: false,
    },
    'AUTOPILOT_AIRSPEED_HOLD_CURRENT': {
        name: 'AUTOPILOT_AIRSPEED_HOLD_CURRENT',
        units: 'Bool',
        settable: false,
    },
} as const;

type PredefinitionName = keyof typeof predefinitions;

type PredefinedValuesResult<SimVars extends PredefinitionName[]> = {
    [VarName in SimVars[number]]: string;
};

type CustomValuesResult<SimVars extends CustomDefinition[]> = {
    [VarName in SimVars[number]['name']]: string;
};

function getPredefinedValue<ReqValues extends PredefinitionName[]>(requestedValues: ReqValues): PredefinedValuesResult<ReqValues> {
    return {} as PredefinedValuesResult<ReqValues>; 
}

function getCustomValues<ReqValues extends CustomDefinition[]>(requestedValues: ReqValues): CustomValuesResult<ReqValues> {
    return {} as CustomValuesResult<ReqValues>; 
}

const predfinedValuesResult = getPredefinedValue([
    'AUTOPILOT_AIRSPEED_ACQUISITION', 
    'AUTOPILOT_AIRSPEED_HOLD_CURRENT',
]);

const customValues = getCustomValues([
    { name: 'CUSTOM_VARIABLE', units: 'degrees' }
]);

console.log(
    predfinedValuesResult.AUTOPILOT_AIRSPEED_ACQUISITION, 
    predfinedValuesResult.AUTOPILOT_AIRSPEED_HOLD_CURRENT, 
);

console.log(
    customValues.CUSTOM_VARIABLE, 
);

// Un-implemented functionality:

const customAndPredefinedValues = getCustomAndPredefinedValues([
    'AUTOPILOT_AIRSPEED_ACQUISITION',
    { name: 'CUSTOM_VARIABLE', units: 'degrees' }
]);

console.log(
    customAndPredefinedValues.AUTOPILOT_AIRSPEED_ACQUISITION, // should autocomplete
    customAndPredefinedValues.CUSTOM_VARIABLE, // should autocomplete
)

Using PredefinedValuesResult and CustomValuesResult alone works fine, but I want to allow the user to specify a list of both custom definitions and referring strings.

Would this be possible?

Answer 1

You could use a conditional type to examine the input array elements and use them directly if they are a key of predefinitions, or to use their name property if they are a CustomDefinition. For example:

type Predefinitions = typeof predefinitions;

declare function getCustomAndPredefinedValues<
    const T extends CustomDefinition | keyof Predefinitions>(
        t: T[]
    ): { [K in (
        T extends keyof Predefinitions ? T :
        T extends CustomDefinition ? T["name"] :
        never
    )]: string };

Now your function behaves as desired:

const customAndPredefinedValues = getCustomAndPredefinedValues([
    'AUTOPILOT_AIRSPEED_ACQUISITION',
    { name: 'CUSTOM_VARIABLE', units: 'degrees' }
]);
/* const customAndPredefinedValues: {
    AUTOPILOT_AIRSPEED_ACQUISITION: string;
    CUSTOM_VARIABLE: string;
} */

Playground link to code