determining the properties of a given data type

I am currently working with a type that I have obtained from a third party source. My goal is to determine the type of a specific property within that type, using TypeScript.

For example:


type GivenType = {
    prop: string;
}

type desiredType = <<GetTypeOf GivenType.prop>>

desiredType === string // true;

I am seeking the correct syntax for 

<<GetTypeOf GivenType.prop>>

After reviewing the TypeScript documentation, I have learned that extracting types is typically performed on objects or interfaces using commands like typeof or returnType.

typescripttypes

Answer №1

To determine the type of a property, you can use the following syntax: 

type SomePropertyType = SomeType["SomeProperty"];

This utilizes an "indexed access operator"; for more information, refer to this specific section in the documentation.

For example, check out this playground snippet:

// number
type LengthPropertyType = String["length"];

// () => number
type GetFullYearType = Date["getFullYear"];

type GivenType = {
    prop: string;
}

// string
type DesiredType = GivenType["prop"];

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