Get Keys of an Object Where Values Are of a Given Type

A common problem in TypeScript is when you want to get the keys of an object where the values are of a given type. For example, let's say you have an object like this:

ts
type Obj = {
  a: string;
  b: string;
  c: number;
  d: number;
};

And we want to only retrieve the keys where the value is a string. So we want to get "a" | "b".

This is deceptively tricky - but let's try it out.

#Quick Solution

This technique uses key remapping to extract only the string keys:

ts
type StringKeys<T> = {
  [K in keyof T]: T[K] extends string ? K : never;
}[keyof T];
 
type StringKeysOfObj = StringKeys<Obj>;
           
type StringKeysOfObj = "a" | "b"

You can make it reusable by providing a generic condition:

ts
type KeysOfValue<T, TCondition> = {
  [K in keyof T]: T[K] extends TCondition
    ? K
    : never;
}[keyof T];
 
type StringKeysOfObj = KeysOfValue<Obj, string>;
           
type StringKeysOfObj = "a" | "b"

#Explanation

Let's look at how I reached this solution, and I'll try to walk you through my thought process.

Generic types like these are easier to understand if you think of them as having a beginning, middle, and end.

In the beginning, we start off with a plain object.

ts
type Obj = {
  a: string;
  b: string;
  c: number;
  d: number;
};

We know we're heading towards a union of keys, so we know keyof will be involved somewhere:

ts
type KeysOfObj = keyof Obj;
// 'a' | 'b' | 'c' | 'd'

You might think - great! We're halfway there. All we'd need now is to exclude the keys that don't have a string value, maybe using the Exclude type helper:

ts
type StringKeys = Exclude<
  KeysOfObj,
  // What do we put here?
  ???
>;

The issue becomes apparent when we try to fill in the second type argument to Exclude. We need the context of the current key to determine whether Obj[Key] is a string or not.

So the 'middle' of our type is going to be about iterating over each key. The whole story is clear - let's implement it.

#Iterating over each key

There are several different ways of achieving this. My favorite is by using an IIMT - an immediately indexed mapped type.

ts
type KeysOfObj = {
        
type KeysOfObj = "hello_a" | "hello_b" | "hello_c" | "hello_d"
  [K in keyof Obj]: `hello_${K}`;
}[keyof Obj];

This lets us create an object using a mapped type, then index into it using keyof Obj to output the value of the object we create.

The benefit of this is that we get access to the current key AND the object in the same scope, so we can use the current key to index into the object.

In this case, we're using it to create a string literal type that prefixes each key with hello_.

But what if we want to do something different depending on the type of the value?

#Conditional types

We can use a conditional type to do this. Conditional types let you express if/else logic with conditionals:

ts
type Tests = [
      
type Tests = [true, true, false, false]
  Obj["a"] extends string ? true : false,
  Obj["b"] extends string ? true : false,
  Obj["c"] extends string ? true : false,
  Obj["d"] extends string ? true : false
];

In this case, we'll let us check if the value of the current key is a string or not. If it isn't, we'll return never, not false.

ts
type KeysOfObj = {
        
type KeysOfObj = "a" | "b"
  [K in keyof Obj]: Obj[K] extends string
    ? K
    : never;
}[keyof Obj];

You'll notice that technically, KeysOfObj should be of type 'a' | 'b' | never | never:

ts
type Tests = [
      
type Tests = ["a", "b", never, never]
  Obj["a"] extends string ? "a" : never,
  Obj["b"] extends string ? "b" : never,
  Obj["c"] extends string ? "c" : never,
  Obj["d"] extends string ? "d" : never
];

But TypeScript will automatically remove never from unions, so we end up with 'a' | 'b'.

ts
type Example = "a" | "b" | never | never;
       
type Example = "a" | "b"

#Making it generic

The final part of the challenge is to turn this into a type helper we can use with any object. We can do that by adding a type argument to StringKeys and removing the hard-coded references to Obj:

ts
type StringKeys<T> = {
  [K in keyof T]: T[K] extends string ? K : never;
}[keyof T];
 
type User = {
  firstName: string;
  lastName: string;
  age: number;
  numberOfCats: number;
};
 
type StringKeysOfObj = StringKeys<User>;
           
type StringKeysOfObj = "firstName" | "lastName"

To make it even more reusable, we can add a second type argument to our helper to let us specify the condition:

ts
type KeysOfValue<T, TCondition> = {
         
type KeysOfValue<T, TCondition> = { [K in keyof T]: T[K] extends TCondition ? K : never; }[keyof T]
  [K in keyof T]: T[K] extends TCondition
    ? K
    : never;
}[keyof T];
 
type StringKeysOfObj = KeysOfValue<User, string>;
           
type StringKeysOfObj = "firstName" | "lastName"
 
type NumericKeysOfObj = KeysOfValue<User, number>;
            
type NumericKeysOfObj = "age" | "numberOfCats"

So there we have it - we've figured out our type helper. We take in an object, run it through an IIMT to iterate over its keys, and use a conditional type to check if the value of the current key matches the condition.