Create a 'key remover' function which can process any generic object
Generics are really interesting in that they're tied to a function's execution context. What that means is that you can make a function, which has one generic locked inside of it, then make another function, in the future, out of that function, with the first generic still locked, and then add another one.
Here's what I mean by that. We want to make a function called makeKeyRemover. Now, makeKeyRemover, is going to return a function that removes keys from an object.
export const makeKeyRemover = () => () => {};const keyRemover = makeKeyRemover(["a", "b"]);const newObject = keyRemover({a: 1, b: 2, c: 3});
The expected case for newObject is that it's going to be an object with just the key of c. In keyRemover, we're making a function. In newObject, we're actually calling the function and using it.
To make this work, first of all, what we're going to do is say, <Key extends string> in makeKeyRemover.
export const makeKeyRemover = <Key extends string>() => () => {};
And then we're going to say that the argument keys is an array of keys. The key's generic has been locked in as a or b.
export const makeKeyRemover = <Key extends string>(keys: Key[]) => () => {};
What we want to do next is take in the object, which is going to be the generic Obj. Then, I'll set the argument obj to Obj.
export const makeKeyRemover = <Key extends string>(keys: Key[]) => <Obj>(obj: Obj) => {};
The generic is now locked in the keyRemover. That means that we need to just remove the keys from the object.
To do that we'll set the return type of this function to be Omit<Obj, Key>. Then we'll return that blank object and set the type to any to quite down an error.
export const makeKeyRemover = <Key extends string>(keys: Key[]) => <Obj>(obj: Obj): Omit<Obj, Key> => { return {} as any; };
What's going on here is that now we've got Omit, which removes keys from an object.
We're removing a and b, the keyRemover locks in that first generic, Key. Then newObject, when we instantiate the second function, locks in the generic, Obj. Now, newObject only has c available to it.
Pretty cool!
Transcript
0:00 Generics are really interesting in that they're tied to a function's execution context. What that means is that you can make a function, which has one generic locked inside of it, then make another function in the future out of that function with the first generic still locked and add another one.
0:16 Here's what I mean by that. Here, we want to make a function called Make Key Remover. Now, Make Key Remover, what it's going to return is a function to remove keys from an object. Here, the expected case for this object is that it's going to be a new object with just the key of C. Here, we're making a function. Here, we're actually calling the function and using it.
0:38 How do we get around to that? First of all, what we're going to do is we're going to say, "Key, extend string here." This is the first function, the key remover one. We're going to say, This key is a...or key array." This is an array of keys. You can see now that the key's generic has been locked in as A or B.
1:01 What we want to do next is we want to take in the object, which is going to be...I'm going to give it a generic of obj. Obj is going to be here. I'll just save it to get the formatting. Obj, as you can see here, it's been locked in. The generic is locked in here into the key remover. That means that we need to just remove the keys from the object.
1:23 We're going to return from this function. I think it's Omit, and then obj, and then key. If I just return this, I need to quieten down the error. What's going on here is that now we've got Omit, which removes keys from an object.
1:41 We're removing A and B. You can see that the key remover locks in that first generic, which is just up here. Then this one, when we actually instantiate the second function, is, it locks in this generic. The new object, it only has C available to it. Pretty cool.
Generics can be 'locked in' by function calls, meaning that generics can be 'curried' through functions. Here, we create a 'key remover' function which can process any generic object.