Mastering TypeScript 3 - Third Edition

Distributed conditional types

When defining a conditional type, we can also use distributive syntax to return one of a number of types. As an example of this, consider the following function definition:

function compareTwoValues(  
    input : string | number | Date, 
    compareTo : string | number | Date ) { 
} 

Here, we have defined a function named compareTwoValues that has two parameters, input and compareTo. Both the input and the compareTo parameters can accept either a string or a number or a Date type. But what if we wanted to apply the following rules:

• If the input parameter is of type Date, then only allow the compareTo parameter to be of type Date
• If the input parameter is of type number, then allow the compareTo parameter to be of type ...