type-guards | Simple utility for runtime type checking | Validation library

I have a question related to user defined type guards, in particular whether it is possible to implement a predicate function that serves as a type guard for a sub field.

Consider the following example:

I use TypeScript in AWS Lambdas. Input objects provided for such Lambdas can be arbitrary. I always need to check if all required fields are provided in the input. I need to provide an error message explicitly saying which fields are missing if this is the case. If all required fields are provided, I need to cast the object into a domain object defined as an interface. Here an example of how this can be done:

Just found following code snippet in https://www.typescriptlang.org/docs/handbook/advanced-types.html#type-guards-and-differentiating-types

Goal: Trying to create a type guard for a custom type.

Here is my custom type:

I'm working with the React Google Login library in TypeScript. It has type bindings for TypeScript, but all the examples are in JavaScript, and I am pretty new to TypeScript.

The setup code looks like this:

According to the TypeScript documentation we can use TypeGuards to differentiate between types.

The documentation gives examples using the is predicate, and using the in operator.

is predicate:

I am reworking an implementation which was previously made using vanilla Javascript. I ran into an interesting situation which I do have a solution for, but for which I thought I had a superior solution which does not appear to work.

Basically, I have an object with two nested objects, their indexes typed as the string literals of their keys, where both objects have some of the other objects keys but not a total overlap. Then, I have a function that can receive either key, and a second key to access one of the values in that object. I created custom type-guards for the key, and created a second set of type-guards to confirm the second key passed was a key of one of the objects. Then, I created a function that takes both keys, and should return an object with correctly matched keys. However, typescript does not seem convinced within the function that I am using my validator function, that the key returned from my function is only one that can definitely access the key of the upper level object.

That a really unclear explanation, and I think some example code will make it better, so here it is:

I'm trying to write a user-defined type guard that tests whether the value it's given has all the properties in a given array.

I'm calling this function hasAll and it's implementation and usage in Javascript would look like this:

The typescript handbook on user defined type guards defines an example type guard as