Recursive Conditional Types

Assume you have a for in loop and suddenly realize that your variable type is string and not a .

So here is the problematic code:

import { cpus } from 'os';



const logicalCoresInfo = cpus();



for (const logicalCoreInfo of logicalCoresInfo) {

  let total = 0;

  for (const type in logicalCoreInfo.times) {

    total += logicalCoreInfo.times[type];  // Darn it, TS is upset!

  }

}

Fix

1. We need to extract the keys inside the logicalCoreInfo.times and create a new type out of it.


2. Then we can utilize which is particularly useful here since we do not know the name of the keys inside the object passed to this utility type. Here you pass logicalCoreInfo to it or any other object, then it iterates through keys to create a new type out of them.
   
   
   

   And -? is there to remove optionality so that we have a string literal union type of all keys. In other word { keyName?: string } will be treated as { keyName: string }.

   
   



3. (TKey extends K ? keyof T[TKey] : never) check if the the current key in the iteration matches the passed key (K), if yes it extracts all keys inside it as a string literal union type and return it. Otherwise it returns nothing.




4. Then if step 3 had no result it will recursively apply this utility type on T[Tkey], this way our utility function works on nested objects as well. This is commonly known as .



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